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Nature Reviews Drug Discovery | Review

Targeting iron metabolism in drug discovery and delivery

Journal name:
Nature Reviews Drug Discovery
Year published:
DOI:
doi:10.1038/nrd.2016.248
Published online

Abstract

Iron fulfils a central role in many essential biochemical processes in human physiology; thus, proper processing of iron is crucial. Although iron metabolism is subject to relatively strict physiological control, numerous disorders, such as cancer and neurodegenerative diseases, have recently been linked to deregulated iron homeostasis. Consequently, iron metabolism constitutes a promising and largely unexploited therapeutic target for the development of new pharmacological treatments for these diseases. Several iron metabolism-targeted therapies are already under clinical evaluation for haematological disorders, and these and newly developed therapeutic agents are likely to have substantial benefit in the clinical management of iron metabolism-associated diseases, for which few efficacious treatments are currently available.

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  1. Systemic iron metabolism.
    Figure 1: Systemic iron metabolism.

    Dietary iron is absorbed by duodenal enterocytes and added to the systemic iron pool upon export via ferroportin (FPN). Iron in the systemic iron pool, which under normal steady-state conditions is mainly bound to transferrin, is primarily utilized for erythropoiesis. Excess systemic iron is stored as ferritin in the liver and, to a lesser extent, in other tissues. The iron utilized in senescent erythrocytes is recycled by macrophages and released, via FPN, back into the systemic iron pool. FPN expression and, consequently, systemic iron availability are inhibited by hepcidin, a peptide primarily produced by the liver. In turn, transcription of the gene encoding hepcidin, HAMP, is stimulated by the presence of iron and inhibited by erythropoietic demand.

  2. Regulation of hepcidin expression in hepatocytes.
    Figure 2: Regulation of hepcidin expression in hepatocytes.

    Bone morphogenetic protein 6 (BMP6) is produced by non-parenchymal cells in the liver in response to iron stores and flux. Binding of BMP6 to the BMP receptor complex on hepatocytes, in conjunction with haemojuvelin (HJV) results in downstream signalling via SMAD1, SMAD5 or SMAD8, which activates SMAD4, which in turn stimulates the transcription and expression of hepcidin (encoded by HAMP). HJV is stabilized by neogenin and cleaved by matriptase 2 (MT2), enabling the fine-tuning of BMP6 signalling. SMAD signalling is additionally regulated by transferrin receptor 2 (TFR2) upon association with human haemochromatosis protein (HFE) and HJV. HFE also interacts with TFR1 to compete with holotransferrin. Consequently, when iron supply is high, HFE is displaced from TFR1 allowing it to associate with TFR2, thereby increasing hepcidin expression via SMAD signalling. Under inflammatory conditions, interleukin-6 (IL-6) and related cytokines bind to the IL-6 receptor, resulting in Janus kinase 1 or 2–signal transducer and activator of transcription (JAK1/2–STAT3) activation and hepcidin expression to restrict iron availability. Finally, developing erythroid cells confer their iron requirements by secreting erythroferrone (ERFE), which inhibits hepcidin expression, via a currently unknown receptor and signalling pathway, to stimulate iron uptake under circumstances of high erythropoietic demand.

  3. Cellular iron trafficking pathways.
    Figure 3: Cellular iron trafficking pathways.

    During iron intake, non-transferrin bound iron (NTBI) can be transported directly into the cell by divalent metal-ion transporter 1 (DMT1), Zrt- and Irt-like protein 14 (ZIP14) and ZIP8. This process occurs after the reduction of Fe(III) to Fe(II) by ferrireductases six-transmembrane epithelial antigen of prostate (STEAP), cytochrome b reductase 1 (DCYTB) or ferric chelate reductase 1. Transferrin-bound iron is taken up via binding to transferrin receptor 1 (TFR1) and endocytosis of the receptor, release and reduction of Fe(III) and transport into the cytoplasm via DMT1. Haemoglobin associates with haptoglobin to allow endocytosis by the scavenger receptor CD163, after which haemoglobin is degraded and haem is transported into the cytosol. Systemic haem is scavenged by complexation with haemopexin and endocytosis via pro-low-density lipoprotein receptor-related protein 1 (LRP1). Haem can be directly transported, also from the endosome, into the cytosol by haem-responsive gene 1 protein homologue (HRG1) and feline leukaemia virus subgroup C receptor-related protein 2 (FLVCR2), where it is processed by haem oxygenase 1 (HO1) and iron is released as Fe(II). Scavenger receptor class A member 5 (SCARA5) internalizes ferritin that consists of light chains, after which iron is liberated and transported to the cytosol. Ferritin composed of heavy chains can be endocytosed by TFR1 (not depicted here). During iron utilization, the labile iron pool (LIP) comprises cytosolic Fe(II), which can be stored in cytoplasmic ferritin or utilized for biochemical processes. Most of these processes take place in the mitochondria, for which iron is supplied via mitoferrin, whereas FLVCR1B allows export of mitochondrially produced haem. The LIP controls the expression of TFR1, DMT1, ferritin and ferroportin (FPN) via the post-transcriptional iron regulatory protein–iron-responsive element (IRP–IRE) regulatory system. Similarly, the transcriptional hypoxia-inducible factor–hypoxia response element (HIF–HRE) regulatory system controls several intracellular processes, including the transcription of TFR1 and DMT1. HIF2α translation is inhibited by IRP1 (also known as cytoplasmic aconitate hydratase); conversely, IRP1 transcription is inhibited by HIF. HIF expression is regulated by prolyl-4-hydroxylase (PHD), which stimulates the degradation of HIFs under conditions of high oxygen. During iron egress, Fe(II) is exported by FPN, followed by oxidation to Fe(III) by the ferroxidases hephaestin or ceruloplasmin, and subsequent binding to transferrin. Intracellular haem can be directly exported via FLVCR1A. Note that not all pathways are present in all cells.

  4. Up- and downregulation of several key players in non-haematological pathologies.
    Figure 4: Up- and downregulation of several key players in non-haematological pathologies.

    Schematic overview illustrating the microenvironmental and systemic changes in iron and iron metabolism-related proteins in selected pathologies in patients: multiple sclerosis (MS; sclerotic plaques); Alzheimer disease (AD; frontal cerebral cortex); Parkinson disease (PD; basal ganglia); atherosclerosis (AS; sclerotic plaques); and cancer (tumour tissue). Upregulation of divalent metal-ion transporter 1 (DMT1) has been observed in MS (preclinical)123, AD (preclinical)255, PD256 and cancer (colorectal)257. Transferrin receptor 1 (TFR1) upregulation has been found in MS258, atherosclerosis259 and cancer (colorectal cancer257, breast cancer95, 101 and glioblastoma94). Normal TFR1 levels have been observed in AD260, 261, whereas normal levels262 and low levels have been reported263, 264 for PD. Increased levels of the macrophage-associated scavenger receptor CD163 have been found in affected tissues of patients with MS265, AD266, PD266, AS267 or cancer (breast cancer268, prostate cancer269 or glioblastoma270). Tissue iron stored as ferritin has found to be increased in MS271, AD272, PD264 and AS259, 273. For cancer, both high levels (glioblastoma)94 and low levels (breast cancer)101 of ferritin have been reported. Similarly, microenvironmental iron deposits have been observed in MS271, AD271, 272, 274, PD256, 264, 274, AS273 and cancer (colorectal)257. Upregulation of ferroportin (FPN) has been demonstrated in tissues of patients with PD264, AS273 or cancer (colorectal257 or breast101), whereas low levels of FPN have been observed in MS (preclinical)123, AD120 and in breast91 and prostate98 cancer. Hepcidin levels are inconsistently associated with (an inverse) FPN expression in diseases. High hepcidin concentrations have been found in MS (preclinical, tissue)123, AS (serum)275, breast cancer (tissue91, 101 and serum276) and prostate cancer (tissue)98, whereas low levels have been observed in the tissues of patients with AD120. Increased systemic ferritin concentrations have found in the cerebrospinal fluid of patients with MS277, AD127 or glioblastoma278, as well as in the serum of patients with AS279 or breast cancer276. However, the levels of serum ferritin have also been found to be normal in patients with AS275 or non-skin cancers280. Systemic iron levels measured by iron concentration and transferrin saturation have been inconsistent: higher than average levels have been found in a meta-analysis in PD281, normal levels have been found in MS282 and AS275, and low levels have been observed in AD283, 284. With regard to cancer, the association with systemic iron levels is unclear: women with high levels of serum iron were at higher risk for developing non-skin cancer, whereas men were at lower risk280.

  5. Proposed mechanisms for targeted drug delivery exploiting iron metabolism-associated cellular targets.
    Figure 5: Proposed mechanisms for targeted drug delivery exploiting iron metabolism-associated cellular targets.

    The natural ligand and the merits of its use as a delivery strategy are presented for each target. Transferrin receptor 1 (TFR1), CD163 and scavenger receptor class A member 5 (SCARA5) facilitate the endocytosis of drugs associated with their natural ligands. Subsequent lysosomal processing would then liberate the targeted drug, which then enters the cytosol by passive diffusion from the lysosome. This process is also likely to occur for haem-targeted constructs upon internalization by pro-low-density lipoprotein receptor-related protein 1 (LRP1). The direct transport of the haem-linked drug into the cytosol by haem-responsive gene 1 protein homologue (HRG1) and/or feline leukaemia virus subgroup C receptor-related protein 2 (FLVCR2) could represent an alternative mechanism of drug uptake into the cytosol. Drug targeting to ferroportin (FPN) has not yet been explored, but forms an interesting approach for directing therapeutics to cells that overexpress FPN, such as some types of tumour cells.

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Affiliations

  1. Department of Polymer Chemistry and Bioengineering, Zernike Institute for Advanced Materials, Faculty of Mathematics and Natural Sciences, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands.

    • Bart J. Crielaard

  2. W. J. Kolff Institute for Biomedical Engineering and Materials Science, University Medical Center Groningen, 9713 AV Groningen, The Netherlands.

    • Bart J. Crielaard

  3. Department of Nanomedicine and Theranostics, Institute for Experimental Molecular Imaging, University Clinic and Helmholtz Institute for Biomedical Engineering, RWTH Aachen University, 52074 Aachen, Germany.

    • Twan Lammers

  4. Department of Pharmaceutics, Utrecht Institute for Pharmaceutical Sciences, Utrecht University, 3584 CG Utrecht, The Netherlands.

    • Twan Lammers

  5. Department of Targeted Therapeutics, MIRA Institute for Biomedical Technology and Technical Medicine, University of Twente, 7522 NB Enschede, The Netherlands.

    • Twan Lammers

  6. Children's Hospital of Philadelphia, Abramson Research Center, Philadelphia, Pennsylvania 19104, USA.

    • Stefano Rivella

Competing interests statement

S.R. has restricted stocks in, is a consultant for and a member of the scientific advisory board of Merganser Biotech. S.R. is also a consultant for Novartis Pharmaceuticals, Bayer Healthcare, and Keryx Biopharmaceuticals, and is a member of the scientific advisory board of Ionis Pharmaceuticals. T.L. is a member of the scientific advisory board of Cristal Therapeutics. B.J.C. declares no competing interests.

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  • Bart J. Crielaard

    Bart J. Crielaard is an assistant professor at the Zernike Institute for Advanced Materials, University of Groningen, the Netherlands. After completing his medical training, he further specialized in biomedical engineering and obtained his Ph.D. in pharmaceutics from Utrecht University, the Netherlands. During his Ph.D. and his postdoctoral fellowship at Weill Cornell Medical College, New York, USA, he developed a particular interest in macrophage biology and iron metabolism, which could be exploited for therapeutic purposes. In his current role as a group leader at the University of Groningen, he is currently investigating this topic and other strategies for the development of new therapeutics. Bart J. Crielaard's homepage.

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  • Twan Lammers

    Twan Lammers obtained a D.Sc. in radiation oncology from Heidelberg University, Germany, in 2008 and a Ph.D. in pharmaceutics from Utrecht University, the Netherlands, in 2009. In the same year, he started the Nanomedicine and Theranostics Group at the Institute for Experimental Molecular Imaging, RWTH Aachen University, Germany. In 2014, he was promoted to full professor at RWTH Aachen University. Since 2012, he has also worked as a part-time assistant professor at the Department of Targeted Therapeutics at the University of Twente, the Netherlands. His primary research interests include drug targeting to tumours, image-guided drug delivery and tumour- targeted combination therapies. Twan Lammers' homepage.

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  • Stefano Rivella

    Stefano Rivella is Professor of Pediatrics with tenure at the Children's Hospital of Philadelphia and University of Pennsylvania, USA, and holds the Kwaku Ohene–Frempong Chair on Sickle Cell Anemia. Rivella characterized the role of seminal factors contributing to morbidity and mortality in various haematological disorders, including hepcidin and ferroportin (which control iron absorption and organ iron distribution), and macrophages (which play an important role in iron recycling and inflammation as well as erythroid proliferation and maturation). Based on these studies, he contributed to the development of novel therapeutics currently in clinical and preclinical trials. Stefano Rivella's homepage.

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