Synthesis of TfR1 & Ferritin Are Reciprocally Regulated
Changes in intracellular iron levels influence the synthesis of both TfR1 and ferritin. When iron is low, the rate of TfR1 synthesis increases while that of ferritin declines. The opposite occurs when iron is abundant and tissue needs have been sated. Control is exerted through the binding of Fe2+ to iron regulatory proteins (IRPs) 1 and 2, cytoplasmic isoforms of the tri carboxylic TCA enzyme aconitase, by hairpin loops structures called iron response elements (IREs). The IREs are located in the 5′ and 3′ untranslated regions (UTRs) of the mRNAs coding for ferritin and TfR1, respectively (Figure 1). Binding of iron-free IRP at the 3′ UTR of the mRNA for TfR1 stabilizes it, thereby increasing TfR1 synthesis, while binding of an IRP to the IRE located at the 5′ UTR of ferritin mRNA blocks translation. When iron levels rise, Fe2+ binds to IRP, completing the assembly of a 4Fe-4S cluster that triggers dissociation of the protein from these DNA hairpins. Once free of IRP, the mRNA encoding ferritin is available to undergo translation. Concurrently, in its IRP-free state the mRNA encoding TfR1 is subject rapid degradation, thereby slowing TfR1 synthesis.
Fig1. Schematic representation of the reciprocal relationship between synthesis of ferritin and the transferrin receptor (TfR1).The mRNA for ferritin is represented on the left, and that for TfR1 on the right of the diagram. At high concentrations of iron, the iron bound to the IRP prevents that protein from binding the IREs on either type of mRNA. The mRNA for ferritin is able to be translated under these circumstances, and ferritin is synthesized. On the other hand, when the IRP is not able to bind to the IRE on the mRNA for TfR1, that mRNA is degraded. In contrast, at low concentrations of iron the IRP is able to bind to the IREs on both types of mRNA. In the case of the ferritin mRNA, this prevents it from being translated. Hence ferritin is not synthesized. In the case of the mRNA for TfR1, binding of the IRP prevents that mRNA from being degraded, enabling it to be translated and TfR1 to be synthesized. IRE, iron response element; IRP, iron regulatory protein.
Hepcidin Is the Chief Regulator of Systemic Iron Homeostasis
The 25-amino acid peptide hepcidin plays a central role in iron homeostasis. Synthesized in the liver as an 84-amino acid precursor (prohepcidin), hepcidin binds to the cellular iron exporter, ferroportin, triggering the latter’s internalization and degradation. The consequent decrease in ferroportin produces a “mucosal block” that lowers iron absorption in the intestine and depresses recycling of the iron liberated as red blood cells turnover (Figure 2). Together, these result in a reduction in circulating iron levels (hypoferremia) as well as reduced placental iron transfer during pregnancy. When plasma iron levels are high, synthesis of hepcidin by the liver increases, reducing both iron absorption and recycling.
Fig2. Role of hepcidin in systemic iron regulation. Hepcidin binds to and triggers the internalization and degradation of ferroportin expressed on the surface of enterocytes and macrophages. This decreases iron absorption from the intestine and inhibits iron release from macrophages, leading to hypoferremia.
Hepcidin Expression Is Influenced by Iron, Erythropoiesis, Inflammation, & Hypoxia
Liver cells monitor iron levels using one of two “iron-sensing complexes” consisting of a homodimer of either TfR1and TfR2 bound to a third transmembrane protein, HFE (Figure 3). HFE protein is a major histocompatibility (MHC) class 1–like molecule that binds β2-microglobulin (a component of class I MHC molecules, not shown in Figure 3) and, normally, TfR1. TfR1 also binds the iron-bound form of transferrin (Tf-Fe), whose binding site overlaps with that for HFE. When iron is abundant and TfFe levels are high, the latter displaces HFE from TfR1. The displaced HFE protein then binds to TfR2, forming a complex that can be further stabilized by association with Tf-Fe. Formation of the HFE-TfR2 complex triggers an intracellular signaling cascade that activates expression of HAMP, the gene encoding hepcidin. It has been observed that mutations in the gene encoding HFE protein are commonly encountered in persons suffering from hereditary hemochromatosis.
Fig3. Regulation of hepcidin gene expression.Tf-Fe (holotransferrin) competes with HFE for binding to TFR1. High levels of Tf-Fe displace HFE from its binding site on TfR1. Displaced HFE binds to TfR2 along with Tf-Fe to signal via the ERK/MAPK pathway to induce hepcidin expression. BMP binds to its receptor BMPR and HJV (coreceptor) to activate R-SMAD. R-SMAD dimerizes with SMAD4, then translocates to the nucleus where it binds to the BMP-RE, resulting in transcriptional activation of hepcidin as shown. IL-6, which is a biomarker of inflammation, binds to its cell surface receptor and activates the JAK-STAT pathway. STAT3 translocates to the nucleus where it binds to its response element (STAT-RE) on the hepcidin gene to induce it. BMP, bone morphogenetic protein; BMPR, bone morphogenetic protein receptor; BMP-RE, BMP response element; ERK-MAPK, extracellular signal-regulated kinase/mitogen-activated protein kinase; HAMP, gene encoding hepcidin antimicrobial peptide (hepcidin); HJV, hemojuvelin; IL-6, interleukin 6; IL-6R, interleukin 6 receptor; JAK, Janus-associated kinase; SMAD, Sma and MAD (mothers against decapentaplegic)-related protein; STAT, signal transduction and activator of transcription; STAT3-RE, STAT 3 response element; TfR1, transferrin receptor 1; TfR2, transferrin receptor 2.
Bone Morphogenetic Proteins Influence Hepcidin Expression
While bone morphogenic proteins (BMPs) act by mechanisms that are distinct from HFE protein, considerable cross-talk occurs between these pathways. For example, the binding affinity of the cell surface receptor for BMP (BMPR) is augmented when BMPR is associated with the coreceptor, hemojuvelin (HJV). The activation of the BMPR-HJV complex triggers the phosphorylation of intra cellular signaling proteins called SMADs, which stimulates the transcription of the gene that codes for hepcidin (see Figure 3).
Inflammatory & Erythropoietic Signals Regulate Hepcidin Levels
During an inflammatory response, synthesis of hepcidin is induced by small secreted proteins called cytokines. Binding of cytokines such as interleukin 6 (IL-6) to their cell surface receptor activates expression of the gene encoding hepcidin via the JAK-STAT (Janus kinase—signal transducer and activator of transcription) pathway (see Figure 3). Inflammation associated cytokines also are thought to trigger the increase in hepcidin levels that accompanies anemia of inflammation (AI). AI manifests as a microcytic, hypochromic anemia that is refractory to iron supplementation.
Hepcidin expression decreases during hypoxia or β-thalassemia. The former is mediated by erythropoietin, whose synthesis is controlled by hypoxia-inducible transcription factors 1 and 2 (HIF-1 and HIF-2). In β-thalassemia, the expression of hepcidin is inhibited by growth differentiation factor 15 (GDF15) and twisted gastrulation 1 (TWSG1), which are secreted by erythroblasts.
