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Transferrin receptor

The transferrin receptor (TfR) is a transmembrane glycoprotein that mediates the cellular uptake of iron by binding to iron-loaded transferrin (holo-transferrin) and facilitating its internalization through receptor-mediated endocytosis, serving as the primary pathway for iron acquisition in vertebrates.^[1] This process is crucial for maintaining iron homeostasis, supporting erythropoiesis, and enabling iron transport across barriers like the blood-brain barrier.^[2] TfRs exist in multiple isoforms, with TfR1 (encoded by the TFRC gene) being the predominant form, a type II membrane protein that forms a homodimer of two 90-97 kDa subunits linked by disulfide bonds, each containing cytoplasmic, transmembrane, and extracellular domains with a butterfly-like structure for high-affinity holo-transferrin binding (Kd ≈ 10⁻⁹ M at neutral pH).^[1] TfR2, encoded by TFR2, shares about 45% homology with TfR1 but exhibits lower affinity for transferrin (25- to 27-fold less) and is mainly expressed in hepatocytes and erythroid precursors, where it regulates hepcidin expression to control systemic iron levels.^[1] Additional isoforms include TfR3 (glyceraldehyde-3-phosphate dehydrogenase, involved in rapid iron uptake under stress like hypoxia) and TfR4 (cubilin, for renal reabsorption), though TfR1 and TfR2 dominate physiological iron handling.^[1] The TFRC gene spans approximately 70 kb with 19 exons, and TfR expression is post-transcriptionally regulated by iron-responsive elements (IREs) in response to cellular iron needs.^[3]^[4] In function, TfR1 binds diferric transferrin at the cell surface, clusters into clathrin-coated pits, and undergoes endocytosis; within the endosome, acidification (pH ≈ 5.5) releases iron from transferrin, which is then reduced to Fe²⁺ and transported into the cytosol via divalent metal transporter 1 (DMT1), while the apo-transferrin-TfR1 complex recycles to the surface.^[5] This cycle allows efficient iron delivery without degrading transferrin, with TfR1 acting as a gatekeeper that adjusts uptake based on iron availability—high TfR1 levels in iron-deficient states enhance import, while iron excess suppresses expression via iron regulatory protein 2 (IRP2).^[5] TfR2 complements this by sensing holotransferrin levels to modulate hepcidin, an iron-regulatory hormone, thus preventing overload or deficiency.^[1] TfRs are ubiquitously expressed in nucleated cells, with highest levels of TfR1 in hemoglobin-synthesizing erythroblasts (up to 1 million copies per cell) and brain capillary endothelial cells for central nervous system iron supply, while TfR2 is primarily expressed in the liver and erythroid tissues.^[2] Dysregulation contributes to disorders: TfR1 mutations cause combined immunodeficiency and microcytic anemia, while TfR2 defects lead to hereditary hemochromatosis type 3; overexpression of TfR1 (often 100-fold higher than normal) in cancers like gliomas promotes tumor growth via excess iron and serves as a therapeutic target for antibody-drug conjugates or nanocarriers that exploit receptor-mediated delivery across the blood-brain barrier.^[1]^[3] Recent advances highlight TfR1's potential in targeted therapies for neurodegenerative diseases, such as Alzheimer's, by shuttling iron chelators or amyloid-clearing agents.^[1]

Overview

Definition and general function

The transferrin receptor (TfR) is a family of proteins that binds iron-loaded transferrin (Tf-Fe) to enable receptor-mediated endocytosis, serving as the primary mechanism for cellular iron uptake in vertebrates. TfR1 and TfR2 are transmembrane glycoproteins, while other members like TfR3 and TfR4 function through alternative surface-binding mechanisms.^[1] This process involves the receptor recognizing and internalizing the Tf-Fe complex at the cell surface, followed by acidification in endosomes to release iron for intracellular use. The general function of TfRs centers on regulating iron entry into cells to sustain systemic iron homeostasis, which is crucial for fundamental biological processes including DNA synthesis, mitochondrial respiration, oxygen transport via hemoglobin, and the activity of iron-dependent enzymes.^[1] By controlling iron availability, TfRs prevent both deficiency, which can lead to anemia, and overload, which risks oxidative damage. TfRs exist in multiple forms that collectively support this role across diverse tissues, though their expression is modulated by cellular iron needs.^[1] TfRs were first identified in the late 1970s as the protein CD71 on reticulocytes, where high expression facilitates massive iron import for hemoglobin production during erythropoiesis. This discovery highlighted TfRs' essential role in averting iron deficiency anemia, particularly in rapidly dividing erythroid cells.^[1] While ubiquitously expressed at low levels in most cells, TfR levels are markedly upregulated in iron-demanding tissues like erythroid precursors to meet heightened requirements for proliferation and differentiation.^[1]

Types of transferrin receptors

Transferrin receptors (TfRs) are classified into four distinct types based on their molecular identities, genetic origins, and functional specializations in iron acquisition. TfR1, also known as CD71, is encoded by the TFRC gene located on chromosome 3q29 and serves as the primary receptor for cellular iron uptake across most tissues.^[6] TfR2 is encoded by the TFR2 gene on chromosome 7q22.1, which produces two main isoforms: the full-length alpha isoform (TfR2α) responsible for membrane-bound iron sensing and a shorter beta isoform (TfR2β) with tissue-specific expression patterns.^[7] In contrast, TfR3 corresponds to glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a ubiquitously expressed glycolytic enzyme that exhibits transferrin-binding capability independent of its primary metabolic role.^[8] TfR4 is identified as cubilin, a large multiligand endocytic receptor encoded by the CUBN gene, which facilitates transferrin interactions through its multi-domain structure.^[9] The key distinctions among these receptors lie in their structural and operational characteristics. TfR1 and TfR2 are classical type II transmembrane glycoproteins that form homodimers and specifically bind iron-loaded transferrin (Tf-Fe) to mediate receptor-dependent endocytosis, with TfR1 exhibiting higher affinity and broader expression than TfR2.^[1] TfR3, functioning as a moonlighting protein, acts primarily as a stress-responsive transferrin binder that localizes to the cell surface under iron-depleting conditions, enabling rapid, low-affinity uptake via alternative endocytic pathways in select cell types such as macrophages.^[10] TfR4, through its cooperation with megalin, supports renal reabsorption of filtered transferrin in proximal tubule cells, relying on its extensive complement of CUB domains for multivalent ligand binding rather than dedicated transmembrane signaling.^[11] Evolutionarily, TfR1 and TfR2 share significant homology in their extracellular domains (approximately 45% sequence identity), reflecting a common ancestral origin within the transferrin receptor family that supports conserved iron-transport mechanisms across vertebrates.^[1] TfR3 and TfR4, however, represent repurposed proteins: GAPDH (TfR3) leverages its ancient enzymatic scaffold for secondary iron-binding roles, while cubilin (TfR4) evolved as part of a broader endocytic complex distinct from the TfR1/TfR2 lineage.^[12] Recent reviews up to 2025 have formalized TfR3 and TfR4 as bona fide transferrin receptors, based on demonstrated binding affinities and functional assays confirming their contributions to iron homeostasis under physiological stress or tissue-specific contexts.^[13]

Molecular structure

Structure of TfR1

The transferrin receptor 1 (TfR1) is a 90 kDa type II transmembrane glycoprotein that functions as a homodimer on the cell surface.^[14] Each monomer consists of a short cytoplasmic tail spanning residues 1–67, which includes a tyrosine-based endocytosis motif (YTRF at positions 20–23), a single transmembrane helix from residues 68–88, and a large extracellular domain encompassing residues 89–760.^[15] The extracellular domain is responsible for ligand binding and adopts a butterfly-like dimeric architecture, with each monomer comprising three globular subdomains: a protease-like domain, an apical domain, and a helical domain that together form a lateral cleft for interaction with transferrin (Tf).^[16] Key structural features of TfR1 include two inter-subunit disulfide bonds at cysteine residues 89 and 98, which covalently link the monomers and contribute to dimer stability, alongside non-covalent interactions that enhance overall integrity.^[17] The protein undergoes N-linked glycosylation at three sites in the extracellular domain—Asn251, Asn317, and Asn727—which are crucial for proper folding, stability, and trafficking to the plasma membrane; mutations at these sites, particularly Asn727, impair surface expression and function.^[18] The Tf-binding site, located in the extracellular region at the interface of the helical and protease-like domains, exhibits pH sensitivity, facilitating high-affinity association with holo-Tf (iron-loaded transferrin) at neutral pH on the cell surface.^[19] Insights into TfR1 structure were first provided by the crystal structure of its ectodomain, resolved at 3.2 Å resolution in 1999 (PDB: 1CX8), revealing the dimeric butterfly shape and the positioning of the Tf-binding clefts.^[20] Subsequent structures, such as the TfR1-holo-Tf complex determined by cryo-EM at 7.5 Å resolution (PDB: 1SUV), illustrate how the receptor engages iron-loaded Tf with high affinity (Kd ≈ 1–5 nM) at physiological pH, while protonation in acidic endosomes (pH ≈ 5.5) triggers conformational changes that promote iron release and receptor recycling.^[21] Dimerization via both covalent disulfides and non-covalent contacts is essential for this high-affinity binding, as monomeric forms exhibit reduced avidity.^[22] Compared to TfR2, TfR1 shares a similar transmembrane orientation but possesses a shorter cytoplasmic tail.^[23]

Structure of TfR2

Transferrin receptor 2 (TfR2), specifically the predominant isoform TfR2α, is an 89 kDa type II transmembrane glycoprotein composed of 801 amino acids that functions as a non-covalently associated homodimer. Its overall topology includes a short cytoplasmic tail spanning residues 1–80, which includes a tyrosine-based endocytosis motif (YQRV); a hydrophobic transmembrane domain from residues 81–104; and a large extracellular domain encompassing residues 105–801 responsible for ligand interactions. This structural organization positions TfR2 primarily on the plasma membrane of hepatocytes and erythroid precursors, where it senses circulating iron levels without efficient internalization.^[24] Key structural features of the extracellular domain include conserved cysteine residues that form disulfide bonds essential for maintaining the protein's folded conformation and stability. Additionally, the extracellular region contains two N-linked glycosylation sites at Asn125 and Asn533, which contribute to proper folding, trafficking, and ligand binding without directly affecting transferrin affinity. The transferrin-binding pocket in the extracellular domain exhibits an altered configuration compared to its homolog, resulting in a lower affinity for holotransferrin (Kd ≈ 20–30 nM at neutral pH), which supports its role in detecting high-iron conditions rather than routine uptake. TfR2 exists in two main isoforms generated by alternative splicing: the full-length TfR2α (801 amino acids), which includes all transmembrane and extracellular components for membrane localization and function; and the truncated TfR2β (100 amino acids), which lacks the transmembrane and extracellular domains, rendering it cytosolic and non-functional for iron uptake. In the cytoplasmic tail, residues 64–82 form a binding domain that interacts with the hereditary hemochromatosis protein HFE, facilitating downstream signaling for hepcidin regulation. Like TfR1, TfR2 shares a homologous extracellular fold consisting of a butterfly-like dimer with helical and protease-like domains, though adapted for liver-specific iron sensing.

Structures of TfR3 and TfR4

Transferrin receptor 3 (TfR3), also known as glyceraldehyde-3-phosphate dehydrogenase (GAPDH), is a multifunctional protein that serves as a non-classical transferrin binder. Unlike the homologous transmembrane TfR1 and TfR2, TfR3 and TfR4 represent moonlighting functions of multifunctional proteins with auxiliary roles in iron handling. It exists as a 37 kDa monomer that assembles into a homotetrameric structure with an approximate molecular weight of 150 kDa. TfR3 lacks a transmembrane domain and functions as a peripheral or soluble protein. Its core structure features a classic NAD-binding Rossmann fold, characteristic of dehydrogenases, with the transferrin-binding site located on the protein surface, enabling moonlighting functions beyond glycolysis.^[25] TfR3 localizes to diverse cellular compartments, including the plasma membrane, cytoplasm, and nucleus, facilitated by post-translational modifications such as lipidation, which anchor it to membranes without integral embedding.^[26] Transferrin receptor 4 (TfR4), identified as cubilin, is a large peripheral membrane protein with a molecular weight of approximately 460 kDa. It comprises 27 complement C1r/C1s, Uegf, and BMP-1 (CUB) domains responsible for ligand binding, interspersed with 8 epidermal growth factor (EGF)-like domains that provide structural spacing and flexibility. Similar to TfR3, TfR4 lacks a transmembrane helix, relying instead on interactions with accessory proteins like megalin and amnionless (AMN) for membrane association and endocytic trafficking. The protein is heavily glycosylated, featuring both N-linked and O-linked modifications that contribute to its stability, solubility, and ligand affinity. Structural studies, including electron microscopy reconstructions, reveal an extended, chain-like architecture that forms a multi-domain "basket" suited for capturing diverse ligands during renal reabsorption.^[27] Recent cryo-EM studies of related complexes (as of 2024) further delineate the modular arrangement of CUB and EGF domains in complex with partners.^[28] In contrast to the transmembrane, dimerizing TfR1 and TfR2, both TfR3 and TfR4 operate as peripheral entities without direct membrane integration, emphasizing auxiliary roles in transferrin handling. TfR3 repurposes the tetrameric glycolytic scaffold of GAPDH for surface transferrin interactions, while TfR4's expansive multi-domain setup supports broad endocytic capture, particularly in specialized tissues like the kidney. These structural distinctions underscore their adaptation for context-specific iron management rather than constitutive uptake.^[23]

Biological functions

Iron uptake and homeostasis

The transferrin receptor (TfR), primarily TfR1, facilitates cellular iron uptake through a receptor-mediated endocytosis pathway that is essential for maintaining iron homeostasis. Iron-loaded transferrin (holo-Tf or Tf-Fe) binds to TfR on the cell surface at neutral pH (approximately 7.4), where the complex is concentrated in clathrin-coated pits for internalization.^[1] This process is energy-dependent, relying on ATP hydrolysis by dynamin to drive vesicle scission from the plasma membrane, forming clathrin-coated vesicles that deliver the complex to early endosomes.^[29] Within the acidified endosome (pH approximately 5.5), protonation induces the release of ferric iron (Fe³⁺) from transferrin, which is then reduced to ferrous iron (Fe²⁺) by the ferrireductase STEAP3.^[30] The Fe²⁺ is subsequently transported across the endosomal membrane into the cytosol via the divalent metal transporter 1 (DMT1), entering the labile iron pool for use in processes such as heme synthesis or storage. Meanwhile, the apo-transferrin (iron-free) remains bound to TfR and is recycled back to the cell surface via recycling endosomes, where at neutral pH it dissociates to allow TfR reuse.^[29]^[1] This cycle enables efficient iron delivery without accumulating free transferrin intracellularly. TfR expression and density play a critical role in systemic and cellular iron homeostasis by inversely correlating with intracellular iron levels: high TfR density occurs during iron deficiency to enhance uptake, while low density in iron overload reduces acquisition to prevent toxicity from free iron, which can generate reactive oxygen species. This regulation supports safe iron storage in ferritin, buffering excess to avoid cellular damage while ensuring availability for essential functions. In humans, the TfR pathway accounts for the majority of cellular iron acquisition, with a daily systemic flux of approximately 20-25 mg to meet demands like erythropoiesis.^[31]^[32]^[33]^[1]

Specific roles of each receptor type

Transferrin receptor 1 (TfR1) predominates in rapidly proliferating cells, such as erythroblasts during erythropoiesis and various tumor cells, where it supports high iron demands for DNA synthesis and cell division.^[34] In addition to facilitating transferrin-bound iron uptake, TfR1 binds H-ferritin, enabling its endocytosis and lysosomal degradation, which modulates cellular iron availability and indirectly influences iron export pathways by altering intracellular iron pools.^[35] Furthermore, TfR1 serves as an entry receptor for pathogens, including Plasmodium vivax merozoites, which exploit it for invasion of reticulocytes during malaria infection.^[36] Transferrin receptor 2 (TfR2), primarily expressed in hepatocytes, functions as a key sensor of circulating iron levels, binding holo-transferrin to detect systemic iron status.^[37] It interacts with the HFE protein to form a complex that, in low-iron states characterized by reduced holo-transferrin saturation, destabilizes and suppresses hepcidin expression, thereby promoting intestinal iron absorption to restore iron balance.^[38] In hepatocytes, TfR2 also contributes to targeted iron delivery to mitochondria, supporting heme biosynthesis and oxidative phosphorylation by channeling transferrin-derived iron through endosomal pathways.^[39] Transferrin receptor 3 (TfR3),^[1] identified as the moonlighting function of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), is induced under hypoxic conditions to enable rapid scavenging of transferrin-bound iron as an adaptive response to oxygen deprivation.^[40] This receptor is internalized via multiple routes, including clathrin-mediated endocytosis and lipid raft-dependent mechanisms, particularly during cellular stress, allowing swift iron acquisition without relying on canonical receptors.^[41] In cancer cells, TfR3 links glycolytic flux to iron metabolism, as GAPDH's dual role enhances iron uptake to fuel energy production and proliferation under hypoxic tumor microenvironments.^[1] Transferrin receptor 4 (TfR4),^[1] corresponding to cubilin, plays a specialized role in the renal proximal tubules by mediating reabsorption of filtered transferrin-iron complexes, thereby preventing urinary loss of this essential resource.^[11] This process is crucial for conserving iron during filtration in the glomerulus, maintaining systemic iron homeostasis.^[1] Endocytosis of cubilin-bound transferrin-iron depends on its co-receptor megalin, which facilitates apical uptake and trafficking to lysosomes for iron release and recycling in kidney cells.^[42]

Regulation

Transcriptional regulation

The transcriptional regulation of transferrin receptors (TfRs) is mediated by specific promoter elements and transcription factors that respond to environmental cues such as hypoxia, cell type, and developmental stage. For TfR1 (encoded by TFRC), the promoter contains hypoxia-inducible factor-1α (HIF-1α) binding sites, enabling induction under hypoxic conditions to enhance iron uptake in low-oxygen environments.^[43] In erythroid cells, GATA-1 and GATA-2 transcription factors bind to regulatory regions of the TFRC promoter, driving expression during erythropoiesis as part of the broader erythroid gene program. Iron excess can repress TFRC transcription through mechanisms involving upstream stimulatory factor (USF) and Sp1 sites in the promoter, although this is secondary to post-transcriptional controls.^[44] TfR2 (encoded by TFR2) expression is driven by GATA-1 binding to consensus sequences in its promoter during erythroid differentiation.^[45] In hepatocytes, TfR2 transcription is regulated by hepatocyte nuclear factor 4α (HNF4α), which maintains basal expression and responds to iron signals for systemic homeostasis.^[46] Unlike TfR1, TfR2 transcription is linked to the cell cycle, peaking during S-phase, and is not directly responsive to iron levels.^[47] TfR3, identified as glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in contexts of iron acquisition, is upregulated by HIF-1 under hypoxia via hypoxia-response elements in its promoter, promoting glycolytic adaptation and iron-related functions.^[48] Its expression is constitutive in glycolytic tissues, reflecting its housekeeping role. TfR4, corresponding to cubilin, exhibits proximal tubule-specific expression driven by hepatocyte nuclear factor-1α (HNF-1α) binding to its promoter, essential for endocytic functions in the kidney.^[49] Cubilin transcription is modulated by vitamin D response elements, linking it to vitamin D metabolism and reabsorption processes.^[50] In general, TfR gene expression is influenced by chromatin accessibility modulated by iron status, with iron deficiency promoting histone acetylation at TFRC regulatory regions to enhance accessibility for transcription factors.^[51] These transcriptional controls complement post-transcriptional mechanisms for fine-tuning receptor levels.

Post-transcriptional regulation

The post-transcriptional regulation of transferrin receptor 1 (TfR1), encoded by the TFRC gene, is predominantly controlled by the iron-responsive element (IRE)/iron regulatory protein (IRP) system at the mRNA level. The TFRC mRNA features five IREs in its 3' untranslated region (UTR), which serve as binding sites for IRP1 and IRP2 under low intracellular iron conditions. Binding of these IRPs to the IREs shields the mRNA from endonucleolytic cleavage, thereby stabilizing it and promoting TfR1 synthesis to facilitate increased iron import.^[52] In contrast, high iron levels prevent IRP binding—through Fe-S cluster assembly in IRP1 and IRP2 degradation—resulting in mRNA destabilization and rapid turnover.^[53] This dynamic control leads to a marked variation in TFRC mRNA half-life, typically ranging from about 45–60 minutes in iron-replete cells to over 10 hours in iron-deficient states.^[54] The underlying iron regulatory loop can be expressed as the IRP binding affinity being inversely proportional to Fe-S cluster assembly:

[\text{IRP binding affinity}] \propto \frac{1}{[\text{Fe-S cluster assembly}]}

This relationship directly governs the observed changes in mRNA stability and TfR1 expression.^[55] Transferrin receptor 2 (TfR2), encoded by the TFR2 gene, lacks IREs in its mRNA and thus escapes IRE/IRP-mediated control, relying instead on protein-level post-transcriptional mechanisms for regulation. TfR2 undergoes lysosomal degradation via the multivesicular body pathway, facilitated by its interaction with the tetraspanin CD81, which promotes sorting and reduces surface receptor levels; siRNA-mediated knockdown of CD81 extends TfR2 half-life and elevates its expression.^[56] This degradation occurs independently of detectable ubiquitination, distinguishing it from other membrane protein turnover pathways.^[57] Palmitoylation further supports TfR2 membrane anchoring and stability, though its precise role in iron-responsive dynamics requires additional investigation.^[1] TfR3, alternatively functioning through glyceraldehyde-3-phosphate dehydrogenase (GAPDH), exhibits post-translational regulation involving iron-dependent membrane recruitment during depletion states, enabling it to bind transferrin and support auxiliary iron uptake when primary receptors are limited.^[58] Phosphorylation events, such as those mediated by Src or AMPK, modulate GAPDH's subcellular localization, including facilitation of nuclear export to enhance its cytosolic and membrane availability under low-iron conditions.^[59]^[60] TfR4, represented by the cubilin-amnionless (AMN) complex, undergoes post-translational trafficking regulation in renal proximal tubules to mediate transferrin endocytosis. Phosphorylation of AMN influences the complex's membrane targeting and endocytic efficiency, with iron excess suppressing endocytosis to curtail uptake and maintain homeostasis.^[61]^[62] This mechanism ensures context-specific iron handling, particularly in epithelial tissues.^[63]

Physiological roles

Role in erythropoiesis

The transferrin receptor 1 (TfR1) plays a pivotal role in erythropoiesis by mediating the uptake of iron-bound transferrin, which is essential for hemoglobin production in developing erythroid cells. In reticulocytes, TfR1 is expressed at approximately 400,000 copies per cell, enabling these cells to acquire the vast majority of iron required for heme synthesis.^[64] This high expression level underscores TfR1's dominance in coordinating iron delivery, as the majority of the iron in erythroid cells is directed toward hemoglobin assembly, the primary oxygen-carrying protein in red blood cells.^[65] Without sufficient TfR1-mediated iron import, erythroid maturation is severely impaired, highlighting its indispensable function in this process. The iron acquired through TfR1 supports heme biosynthesis by providing ferrous iron to the mitochondrial enzyme 5-aminolevulinate synthase 2 (ALAS2), the rate-limiting step in the erythroid-specific heme pathway.^[66] Upon endocytosis of the transferrin-TfR1 complex in early endosomes, iron is released under acidic conditions and transported to mitochondria via mechanisms such as the "kiss-and-run" pathway, where endosomes transiently fuse with the mitochondrial surface to deliver iron directly.^[67] TfR1 exhibits a high recycling efficiency in erythroid cells, allowing rapid and repeated iron acquisition to meet the intense demands of hemoglobin synthesis during a short developmental window.^[67] TfR1 expression varies across erythroid developmental stages, peaking in basophilic erythroblasts where iron requirements are maximal for early hemoglobin accumulation.^[68] As erythroblasts progress to polychromatophilic and orthochromatic stages, TfR1 levels gradually decline, and it is ultimately downregulated and shed during reticulocyte maturation into mature red blood cells, coinciding with enucleation and the cessation of protein synthesis.^[69] This downregulation is linked to apoptotic signals that facilitate nuclear extrusion and membrane remodeling, ensuring that mature erythrocytes, which lack organelles, no longer require active iron uptake.^[67] Genetic evidence confirms TfR1's critical role, as homozygous knockout of the TFRC gene in mice results in embryonic lethality around E12.5, primarily due to profound anemia from failed erythrocyte development and insufficient hemoglobin production. These embryos exhibit arrested erythropoiesis and neural defects, demonstrating that TfR1-mediated iron delivery is vital from early gastrulation onward.^[70]

Role in tissue-specific iron distribution

Transferrin receptor 2 (TfR2), predominantly expressed in hepatocytes, plays a critical role in hepatic iron sensing by detecting plasma transferrin-iron (Tf-Fe) saturation levels to modulate hepcidin expression through interactions with HFE protein.^[38] This sensing mechanism ensures appropriate hepcidin upregulation in response to elevated iron availability, thereby regulating systemic iron export from enterocytes and macrophages.^[71] Additionally, TfR2 facilitates the delivery of iron to hepatocytes, where it is stored primarily as ferritin, maintaining the liver's role as a central iron reservoir.^[72] In the kidney, TfR4, identified as cubilin in complex with megalin, is essential for the reabsorption of nearly 99% of filtered Tf-Fe in the proximal tubules, preventing urinary iron loss and averting hypoferremia.^[1] This process involves receptor-mediated endocytosis, allowing efficient recycling of transferrin-bound iron back into the circulation and supporting overall iron homeostasis without significant contribution to cellular storage in renal tissue.^[73] At the blood-brain barrier, TfR1 enables the transport of iron into the central nervous system, providing neurons with essential iron for processes such as myelin synthesis in oligodendrocytes and dopamine production via tyrosine hydroxylase.^[74] TfR1 expression on brain endothelial cells is high, supporting efficient yet controlled iron uptake to meet precise neuronal demands and avoid potential oxidative damage from excess iron.^[75] In other tissues, such as skeletal muscle, TfR3—associated with glyceraldehyde-3-phosphate dehydrogenase (GAPDH)—facilitates rapid Tf-Fe uptake during hypoxic conditions, enabling a swift metabolic response to oxygen deprivation without altering baseline iron distribution.^[1] Collectively, these receptor-specific functions establish iron concentration gradients across organs, ensuring targeted allocation for storage in the liver, conservation in the kidney, controlled entry in the brain, and adaptive responses in muscle.^[76]

Role in diseases

Iron overload and deficiency disorders

The transferrin receptor 2 (TfR2) plays a critical role in hepatic iron sensing and hepcidin regulation, and its mutations are associated with type 3 hereditary hemochromatosis, a subtype characterized by progressive iron overload primarily in the liver. For instance, the Y250X truncation mutation in the TFR2 gene disrupts TfR2 function, leading to impaired hepcidin expression and excessive intestinal iron absorption, resulting in hepatic iron accumulation and potential fibrosis. This mutation has been identified in families with early-onset hemochromatosis, confirming its causal role through mouse models that recapitulate the iron overload phenotype. Similarly, defects in the interaction between HFE protein and transferrin receptor 1 (TfR1) contribute to the more common type 1 hereditary hemochromatosis; HFE normally competes with transferrin for TfR1 binding to modulate iron uptake signaling, but common mutations like C282Y prevent this interaction, reducing hepcidin levels and promoting systemic iron overload. These disruptions highlight how TfR1 and TfR2 facilitate iron homeostasis by linking transferrin saturation to regulatory pathways. In anemia of chronic disease (ACD), also known as anemia of inflammation, inflammatory cytokines such as IFN-γ and LPS downregulate TfR1 expression on macrophages, contributing to iron sequestration within these cells and limiting iron availability for erythropoiesis. This downregulation, combined with cytokine-induced hepcidin upregulation (primarily via IL-6), degrades ferroportin on macrophages, further trapping recycled iron and exacerbating the hypoferremic state typical of ACD. The resulting functional iron deficiency impairs hemoglobin synthesis despite adequate total body iron stores, distinguishing ACD from absolute iron deficiency anemia. Cubilin, a multi-ligand endocytic receptor in renal proximal tubules, mediates the reabsorption of filtered transferrin; its deficiency underlies Imerslund-Gräsbeck syndrome, a rare autosomal recessive disorder featuring selective vitamin B12 malabsorption, megaloblastic anemia, and proteinuria due to impaired endocytosis of low-molecular-weight proteins including transferrin. The proteinuria involves low-molecular-weight proteins including transferrin, but the primary hematologic manifestation is megaloblastic anemia due to vitamin B12 malabsorption. Studies have linked rare germline variants in the TFRC gene (encoding TfR1) to congenital microcytic anemia, characterized by hypochromic red cells and partial resistance to iron therapy, expanding the spectrum of transferrin receptor-related iron disorders.

Involvement in cancer and infections

The transferrin receptor 1 (TfR1) is frequently overexpressed in malignant cells to meet the elevated iron demands associated with rapid proliferation. This upregulation occurs in a wide range of solid tumors, including breast and lung cancers, where TfR1 expression supports tumor cell growth and survival. High levels of TfR1 have been linked to poor clinical outcomes, such as reduced overall survival in patients with breast cancer and osteosarcoma.^[34]^[77]^[78]^[79] In cancer, TfR1 contributes to tumor progression beyond iron transport by influencing vascularization. Elevated TfR1 expression correlates with increased vascular endothelial growth factor (VEGF) levels, promoting angiogenesis and metastasis in aggressive tumors like osteosarcoma. This association underscores TfR1's role in creating a supportive microenvironment for cancer expansion.^[80] TfR1 also serves as an entry receptor exploited by certain pathogens during infections. For influenza A virus, the viral hemagglutinin (HA) protein binds to sialic acids on TfR1, facilitating endocytosis and uncoating within host cells. Bacterial pathogens such as Neisseria meningitidis and Neisseria gonorrhoeae bind directly to host transferrin using their own transferrin-binding proteins (TbpA and TbpB) to acquire iron essential for survival. Similarly, the malaria parasite Plasmodium vivax (closely related to P. falciparum) invades reticulocytes via a TfR1-transferrin bridge, where the parasite protein PvRBP2b binds TfR1 to enable host cell entry. However, recent studies indicate that this interaction may not be essential for invasion by all strains, suggesting alternative pathways.^[81]^[82]^[83]^[84]^[85] Pathogens hijack the TfR1 endocytosis machinery to enhance nutrient acquisition and invasion efficiency. In malaria, P. vivax exploits this pathway to increase iron uptake during reticulocyte targeting, thereby supporting parasite replication. These mechanisms allow infectious agents to subvert host iron homeostasis for their benefit.^[85] Studies have highlighted the role of transferrin receptor 2 (TfR2) in inflammatory conditions, particularly in macrophages during arthritis. Deletion of TfR2 in macrophages promotes pro-inflammatory M1-like polarization, exacerbating joint inflammation and bone erosion in models of inflammatory arthritis, indicating that TfR2 normally suppresses excessive immune activation in these cells.^[86]

Therapeutic targeting

Applications in drug delivery

The transferrin receptor 1 (TfR1), highly expressed on brain endothelial cells forming the blood-brain barrier (BBB), serves as a key target for receptor-mediated transcytosis (RMT) to facilitate the delivery of biologics into the central nervous system.^[87] This endogenous pathway, which normally transports iron-bound transferrin across the BBB, can be hijacked by conjugating therapeutic molecules to anti-TfR1 antibodies or transferrin itself, enabling enhanced brain penetration without disrupting iron homeostasis.^[88] Pioneering work utilized the OX26 monoclonal antibody, which binds rat TfR1 without competing with transferrin, to mediate transcytosis of fused therapeutics in preclinical models.^[89] Similarly, transferrin conjugates have been employed to shuttle enzymes and antibodies across the BBB. Recent 2025 advances incorporate bispecific antibodies targeting both TfR1 and CD98hc, a BBB-enriched transmembrane protein, resulting in faster initial uptake via TfR1 and prolonged brain retention through CD98hc-mediated stabilization, achieving up to several-fold higher parenchymal exposure compared to monovalent TfR1 binders.^[90] The underlying mechanism involves saturable RMT, reflecting the limited receptor capacity at the BBB.^[91] To evade lysosomal degradation during endosomal trafficking, pH-insensitive mutants of anti-TfR1 antibodies have been engineered, maintaining binding stability in acidic compartments and promoting apical release into the brain interstitium rather than recycling or degradation.^[92] Representative examples include the ch128.1 IgG3 anti-TfR1 antibody conjugated to anti-amyloid-β therapeutics, which demonstrated a 10-fold increase in brain uptake for Alzheimer's disease models, enhancing amyloid clearance without peripheral toxicity.^[1] For gene therapy, H-ferritin nanocages, which naturally bind TfR1 via their ferroxidase domain, have encapsulated siRNA or CRISPR components, enabling efficient BBB crossing and targeted nucleic acid delivery in vitro and in vivo.^[93] Key challenges include the "peripheral sink" effect, where high-affinity binders sequester on TfR1-expressing erythrocytes and other peripheral tissues, reducing BBB availability.^[94] 2025 studies emphasize optimizing antibody affinity with dissociation constants (Kd) of 10–100 nM to balance brain uptake efficiency and minimize off-target binding, as affinities below 10 nM exacerbate peripheral sequestration while higher values diminish transcytosis.^[95]

Use in cancer therapies

The transferrin receptor 1 (TfR1), often overexpressed on cancer cells to meet elevated iron demands, serves as a promising target for antibody-drug conjugates (ADCs) in oncology. ADCs leveraging anti-TfR1 monoclonal antibodies, such as CX-2029—a probody drug conjugate linking an anti-CD71 antibody to the auristatin derivative monomethyl auristatin E (MMAE)—exploit receptor-mediated endocytosis to deliver cytotoxic payloads selectively to tumor cells. In phase I clinical trials, CX-2029 demonstrated antitumor activity in patients with advanced solid tumors, with objective responses observed at doses up to 3 mg/kg; however, the program was terminated in 2023 with no further development. The trial showed a manageable safety profile characterized by primarily hematologic toxicities.^[96]^[97]^[98] Similarly, the humanized anti-TfR1 antibody 3TF12, originally developed as a single-chain variable fragment (scFv), has been adapted into bivalent formats and conjugated to payloads for enhanced cytotoxicity against iron-dependent leukemias and lymphomas, inhibiting cell proliferation by blocking transferrin binding and promoting receptor internalization.^[99] Immunotoxins represent another strategy harnessing TfR1 for cancer therapy, particularly in CD71-positive hematologic malignancies like lymphomas. These agents fuse anti-TfR1 antibodies or ligands to bacterial toxins, such as truncated diphtheria toxin, enabling receptor-mediated uptake via the endocytic pathway, followed by toxin translocation to the cytosol to inhibit protein synthesis and induce apoptosis. For instance, TfR-directed immunotoxins have shown potent cytotoxicity against primary lymphoid malignant cells, including Burkitt's lymphoma lines, with IC50 values in the nanomolar range due to efficient internalization and lysosomal escape. Preclinical studies highlight their specificity for proliferating cells overexpressing TfR1, minimizing off-target effects on normal tissues with lower receptor density.^[100]^[101]^[102] Nanocarrier systems, including TfR1-targeted liposomes, enhance the delivery of chemotherapeutic agents like doxorubicin to tumors while reducing systemic toxicity. These liposomes are surface-modified with transferrin or anti-TfR1 antibodies to facilitate receptor-mediated endocytosis, leading to 5–10-fold greater drug accumulation in tumor tissues compared to non-targeted formulations in preclinical models of breast and cervical cancers. In multidrug-resistant cell lines, such as HeLa and SBC-3/ADR, TfR-targeted doxorubicin-loaded liposomes achieved up to 7-fold higher cellular uptake and restored sensitivity to the drug, correlating with improved tumor regression in xenograft models. This approach capitalizes on TfR1's role in rapid endocytosis, allowing sustained payload release within acidic endosomes.^[103]^[104]^[105] Recent advances in TfR1-targeted therapies include the development of single-domain antibodies (sdAbs) with high affinity and cross-species reactivity, enabling more stable and versatile formats for cancer applications. In 2025 studies, novel sdAbs targeting a unique TfR1 epitope demonstrated superior internalization and payload delivery in preclinical tumor models, offering potential for bispecific constructs that engage immune effectors.^[106] Furthermore, anlotinib, which modulates TfR1-dependent pathways, combined with immune checkpoint inhibitors such as anti-PD-1 antibodies, has shown synergistic effects by boosting T-cell infiltration in hepatocellular carcinoma models, as evidenced by preclinical data where this combination amplified immunotherapy responses.^[107]^[108] As of 2025, TfR1-targeted therapies remain investigational, with no FDA-approved options for cancer treatment. These innovations underscore TfR1's evolving role in precision oncology, with ongoing trials exploring multimodal regimens.^[109]

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