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Review
. 2016 Mar;41(3):274-286.
doi: 10.1016/j.tibs.2015.11.012. Epub 2015 Dec 23.

Regulators of Iron Homeostasis: New Players in Metabolism, Cell Death, and Disease

Alexander R Bogdan  1 , Masaki Miyazawa  1 , Kazunori Hashimoto  1 , Yoshiaki Tsuji  2
Affiliations
Review

Regulators of Iron Homeostasis: New Players in Metabolism, Cell Death, and Disease

Alexander R Bogdan et al. Trends Biochem Sci. 2016 Mar.

Abstract

Iron is necessary for life, but can also cause cell death. Accordingly, cells evolved a robust, tightly regulated suite of genes for maintaining iron homeostasis. Previous mechanistic studies on iron homeostasis have granted insight into the role of iron in human health and disease. We highlight new regulators of iron metabolism, including iron-trafficking proteins [solute carrier family 39, SLC39, also known as ZRT/IRT-like protein, ZIP; and poly-(rC)-binding protein, PCBP] and a cargo receptor (NCOA4) that is crucial for release of ferritin-bound iron. We also discuss emerging roles of iron in apoptosis and a novel iron-dependent cell death pathway termed 'ferroptosis', the dysregulation of iron metabolism in human pathologies, and the use of iron chelators in cancer therapy.

Keywords: NCOA4; SLC39; ferritin; ferroportin; ferroptosis; iron chelator.

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Figures

Figure 1
Figure 1. New Mechanisms Regulating Intracellular Iron Metabolism
The ZRT/IRT-like protein (ZIP) family transporters, ZIP8 and ZIP14, were recently identified as crucial for transporting non-transferrin bound iron (NTBI) after reduction of NTBI by prion protein (PRNP). In the acidic endosome, Fe3+ is released from transferrin (Tf) and free Fe3+ is reduced to Fe2+ by six-transmembrane epithelial antigen of prostate 3 (STEAP3) and transported to the cytoplasm by divalent metal transporter 1 (DMT1) and ZIP8/14. Poly-(rC)-binding protein 1 (PCBP1) and PCBP2 are cytosolic iron chaperones that deliver Fe2+ to apo-proteins (metallation), such as hypoxia-inducible factor (HIF) prolyl hydroxylases), ferroportin (iron export), and ferritin (oxidation to Fe3+ and storage). Nuclear receptor coactivator 4 (NCOA4)-mediated autophagy of iron-loaded ferritin releases iron for utilization in cellular processes (see text).
Figure 2
Figure 2. Intracellular Iron and Antioxidant Depletion Are Crucial Mediators of Ferroptosis
Ferroptosis appears to occur as a consequence of two cellular events: disruption of the cellular antioxidant pool and an increase of intracellular iron, leading to a buildup of toxic lipid peroxides. Chemical compounds such as erastin and sorafenib as well as p53 signaling disrupt cysteine transport, leading to depletion of cellular antioxidant glutathione (GSH). GSH depletion also occurs during ischemia/reperfusion (I/R) injury, wherein oxidative stress overwhelms the cellular antioxidant capacity. Loss of GSH inhibits glutathione peroxidase 4 (GPX4), a key lipid reduction enzyme. Increase in intracellular iron by erastin treatment, iron regulatory protein 2 (IRP2) activity, and iron-loaded transferrin after I/R injury also contribute to the accumulation of lipid peroxides and death by ferroptosis. Inhibitors of ferroptosis function by detoxifying lipid peroxides (ferrostatin 1) or decreasing intracellular iron [iron chelators, heat shock protein B1 (HSBP1)]. The pathway from lipid peroxides to ferroptotic cell death is still not determined; future studies should show if lipid peroxides are the terminal effectors of ferroptosis or if additional ferroptosis mediator/effector molecules are downstream.
Figure 3
Figure 3. The Ferroportin–Hepcidin Axis Is Altered in Cancers
Normal breast and prostate cells (right) contain relatively low intracellular iron as a result of high ferroportin, low hepcidin, and low TfR1 expression. By contrast, breast and prostate cancer cells (left) often have elevated TfR1 as well as low ferroportin expression because of hypermethylation of the ferroportin promoter which inhibits Fpn transcription factors (Nrf2, MZF1). In addition, post-translational repression of Fpn by tumor- and liver-derived hepcidin further blocks iron efflux. Consequently, these cancer cells can aberrantly accumulate intracellular iron, leading to increased cell proliferation and an aggressive cancer phenotype.
Figure 4
Figure 4. Both Iron Excess and Deficiency Are Deleterious to Cells
Cell death by iron overload (left) is thought to be the result of reactive oxygen species (ROS) generation, particularly in the lysosome. Iron-overloaded cells accumulate iron inside lysosomes as a consequence of the degradation of iron-loaded ferritin. Inside the lysosome, iron can cycle between Fe3+ and Fe2+, generating ROS and leading to lysosomal membrane permeabilization and DNA damage. Cathepsin and other lysosomal proteases induce release of mitochondrial cytochrome c (Cyt. c) and other pro-apoptotic proteins, leading to downstream caspase activation and apoptosis. Iron chelation (right) has multiple antiproliferative/pro-death effects on the cell. Iron chelators can induce mitophagy, cell cycle arrest, and apoptosis through various other mechanisms, including blocking DNA biosynthesis through inhibition of ribonucleotide reductase, downregulation of cyclins and cyclin-dependent kinases (CDKs), induction of the tumor suppressor p53, downregulation of the apoptosis inhibitor Bcl-2, and upregulation of apoptosis activator Bax.
Figure I
Figure I. Mechanisms of Intracellular and Systemic Iron Homeostasis
(A) Intracellular iron homeostasis is predominantly controlled by the post-transcriptional control of iron metabolism genes via the iron responsive element–iron regulatory protein (IRE–IRP) system. The iron-sensitive IRE–IRP interaction regulates the translation rate or mRNA stability of mRNAs depending upon the location of the IRE in the 5′- or 3′-untranslated region (UTR). (B) Systemic iron homeostasis is regulated by the circulating peptide hormone hepcidin, which binds to ferroportin (Fpn) on the plasma membrane and induces Fpn internalization and degradation on various cell types.
Figure I
Figure I. Hexadentate and Tridentate Chelators Sequester Iron with Different Chemistries
Deferoxamine (DFO) forms six bonds with a central iron atom, rendering the Fe3+ inert. Tridentate chelators, such as Dfx and Dp44mT, only form three bonds with iron, allowing iron reduction and Fenton chemistry with subsequent reactive oxygen species (ROS) generation.
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