Complex regulation of iron metabolism in health and disease

[madman]

Abstract

The peptide hormone hepcidin is central to the regulation of iron metabolism, influencing the movement of iron into the circulation and determining total body iron stores. Its effect on a cellular level involves binding ferroportin, the main iron export protein, preventing iron egress and leading to iron sequestration within ferroportin-expressing cells. Hepcidin expression is enhanced by iron loading and inflammation and suppressed by erythropoietic stimulation. Aberrantly increased hepcidin leads to systemic iron deficiency and/or iron restricted erythropoiesis as occurs in anemia of chronic inflammation. Furthermore, insufficiently elevated hepcidin occurs in multiple diseases associated with iron overload such as hereditary hemochromatosis and iron loading anemias. Abnormal iron metabolism as a consequence of hepcidin dysregulation is an underlying factor resulting in pathophysiology of multiple diseases and several agents aimed at manipulating this pathway have been designed, with some already in clinical trials. In this chapter, we assess the complex regulation of hepcidin, delineate the many binding partners involved in its regulation, and present an update on the development of hepcidin agonists and antagonists in various clinical scenarios.




2. Regulation of iron metabolism

2.1 Systemic iron metabolism regulation

2.2 Cellular regulation of iron metabolism
(1) Duodenal enterocytes
(2) Reticuloendothelial macrophages
(3) Erythroblasts


3. Physiological regulation of hepcidin expression


4. Hepcidin regulation by inflammation


5. Hepcidin regulation by erythropoiesis


6. Hepcidin regulation and hepcidin-independent regulation of iron absorption by hypoxia


7. Hepcidin-ferroportin axis regulates iron flows


8. Hepcidin-ferroportin axis in disease

8.1 Hereditary hemochromatosis

8.2 Iron-loading anemias

8.3 Anemia of chronic inflammation

8.4 Polycythemia vera


9. Targeting the hepcidin:ferroportin axis for therapeutic purposes




10. Conclusion

The discovery of hepcidin as a central regulator of iron metabolism and erythroid regulation of hepcidin by ERFE enabled a mechanistic exploration of aberrant iron metabolism in many hematopoietic and non-hematopoietic diseases. This enhanced understanding has within a relatively short timeframe lead to the development of novel compounds manipulating this pathway to support both exogenous agonist and antagonist function with multiple agents already undergoing clinical trials for several indications.

 

[madman]

Fig. 1 Hepcidin is central to the regulation of iron metabolism. (A) Systemically, hepcidin is a negative regulator of iron flows such that increased hepcidin synthesis (which mainly occurs in the liver) leads to hypoferremia by decreasing iron absorption in the duodenum, iron recycling from splenic macrophages, and iron release from hepatocyte stores. (B) The mechanism of action of hepcidin involves binding to and occluding ferroportin, induction of ferroportin ubiquitination, followed by endocytosis and lysosomal degradation of the ferroportin:hepcidin complex. Fe-Tf¼transferrin-bound iron; FPN1¼ferroportin1; Ub¼ubiquitination.

 

[madman]

Fig. 2 Cellular iron metabolism. Intracellular iron homeostasis is balanced by coordinated iron uptake, utilization, storage and export. The three main cells of interest include duodenal enterocytes (involved in systemic iron absorption), reticuloendothelial macrophages (involved in systemic iron recycling), and erythroblasts (main location of systemic iron utilization for hemoglobin synthesis during erythropoiesis). (A) Duodenal enterocyte: Absorbed inorganic ferric iron (Fe3+) must be first converted to ferrous iron (Fe2+) via ferrireductase Dcytb and subsequently taken up by iron importer DMT1. Once inside the cell, iron is shuttled to ferritin via iron chaperones PCBP1/2 and stored there or shuttled out of ferritin by NCOA4 for export via FPN1. During iron export, Fe2+ must be oxidized to Fe3+ by HEPH or CP and loaded onto TF for transport in the circulation. Hepcidin prevents iron export at the basolateral cell membrane and results in ferritin iron accumulation within the enterocyte. (B) Macrophage: Splenic and liver macrophages are specifically equipped with mechanisms to enable direct erythrophagocytosis, uptake of Hb:HP complexes via CD163, and heme:HPX complexes via CD91. The heme extracted from these pathways is processed by HMOX1 to liberate iron that is then either incorporated into ferritin or exported from the cell via FPN1 and loaded onto TF for delivery to iron-requiring cells. (C) Erythroblast: Iron loaded TF binds to TFR1 on the surface of cells with erythroblasts expressing the highest concentration of TFR1 relative to other cells in light of their high iron requirements. These complexes localize to clathrin-coated pits that invaginate to form specialized endosomes where proton pumps decrease the pH and transported Fe3+ is reduced by STEAP3 for export from the endosome via DMT1. Erythroblasts shuttle much of their iron to the mitochondria by an incompletely understood mechanism where it is incorporated into protoporphyrin. FPN1 is also expressed on erythroblasts but purpose of iron export in erythroblasts is incompletely understood. Finally, iron loaded TF also binds TFR2 which is thought to function as an iron sensor to coordinate iron supply with erythropoietic output by modulating EPOR localization and consequently EPO responsiveness; a detailed mechanistic understanding of TFR2’s role in erythropoiesis (DMT1¼divalent metal transporter 1; Dcytb¼duodenal cytochrome B reductase; FPN¼ferroportin 1; HEPH¼hephaestin; CP¼ceruloplasmin; TF¼transferrin; Fe3+ ¼ferric iron; Fe2+ ¼ferrous iron; Hb¼hemoglobin; HP¼haptoglobin; HPX¼hemopexin; CD91 and 169¼cluster of differentiation 91 and 163; HMOX1¼heme oxygenase 1; TFR1 and 2¼transferrin receptor 1 and 2; EPO¼erythropoietin; EPOR¼EPO receptor; PCBP1¼poly(rC)-binding protein 1; NCOA4¼nuclear receptor coactivator 4; pSTAT5¼phosphorylated signal transducer and activator of transcription 5; pat¼phosphorylated protein kinase B).

 

[madman]

Fig. 3 Hepcidin regulation. (A) Systemically, hepcidin is stimulated by transferrin-bound iron in the circulation and liver iron stores as well as by systemic inflammation and suppressed by enhanced erythroid activity. (B) Regulation of hepcidin expression in the hepatocyte involves JAK-STAT signaling as a consequence of IL-6 receptor stimulation and SMAD signaling as a consequence of a BMP receptor complex stimulation. IL-6 and BMP2/6 binding their receptors, respectively, leads to stimulation of hepcidin expression. Stimulation of erythropoiesis leads to the expression and secretion of erythroferrone which sequesters BMP2/6 to suppress SMAD signaling, decreasing hepcidin. Additional coregulation via matriptase-2 and hemojuvelin as well as systemic iron sensing byTFR1, HFE, and TFR2 enhance BMP receptor stimulation and increase hepcidin expression. Fe-Tf¼transferrin-bound iron; IL-6¼interleukin 6; HAMP ¼gene name for hepcidin; ERFE¼erythroferrone; LSEC¼liver sinusoidal endothelial cell; TFR1/2¼transferrin receptor 1 and 2; HFE¼homeostatic iron regulator; HJV¼ hemojuvelin; MT-2¼matriptase 2; JAK-STAT¼janus kinase and signal transducer and activator of transcription; SMAD¼small mothers against decapentaplegic.

 

[madman]

Fig. 4 Effects of inflammation of iron recycling. Under normal conditions, iron recycling from multiple sources within macrophages leads to export of iron via ferroportin back into the circulation where it is loaded onto transferrin and delivered to cells with iron requirements (e.g., for hemoglobin synthesis in erythroblasts during erythropoiesis in the bone marrow). Increased hepcidin in states associated with chronic inflammation lead to binding to and occlusion of the ferroportin channel, preventing iron egress from cells involved in iron recycling (e.g., splenic red pulp macrophages), leading to the accumulation of iron within cellular ferritin core, and causing decreased iron-bound transferrin (low transferrin saturation). This decreased availability of iron for erythropoiesis results in anemia of chronic inflammation. Fe¼iron; FPN¼ferroportin 1; TF¼transferrin; Hb¼ hemoglobin; HP¼haptoglobin; HPX¼hemopexin; PCBP1¼poly(rC)-binding protein 1; NCOA4¼nuclear receptor coactivator 4.

 

[madman]

Fig. 5 Effects of hepcidin-mimetic on erythropoiesis in polycythemia vera. In polycythemia vera, where hepcidin levels are low, iron recycling within macrophages leads to export of iron via ferroportin back into the circulation where it is loaded onto transferrin and delivered to the bone marrow for hemoglobin synthesis in erythroblasts during erythropoiesis. Exogenous hepcidin-mimetic agents or inducers of endogenous hepcidin lead to binding to and occlusion of the ferroportin channel, preventing iron egress from macrophages involved in iron recycling, leading to the accumulation of iron within cellular ferritin core, and causing decreased iron availability for erythropoiesis, suppressing RBC production in the bone marrow. RBC¼red blood cell; Fe¼iron; TF¼transferrin; FPN¼ferroportin 1.

 

[madman]

Table 1 Hepcidin mimetics in development for PV patients.