TFR1 (TFRC): a Stimulator of Fe Transport Affects Anemia, Neurodegenerative Disorders and Cancers!

1. What is TFR1?

1.1 TFR1 Structure

Transferrin receptor protein 1 (TFR1) is a crucial type II transmembrane protein that regulates intracellular iron transport across the membrane. TFR1 comprises of two homodimeric subunits, which are linked by disulfide bonds. Each monomer consists of a short N-terminal region, a single transmembrane region, and a large extracellular C-terminal region. This C-terminal region, serving as an additional functional region, contains the same protein present in transferrin (Tf) (Figure 1) ^[2-4]. At present, two similar transferrin receptors, TFR1 and TFR2, have been identified. Under normal physiological conditions, TFR1 interacts with Tf to facilitate iron uptake. This bound form is the main way of iron presence in the blood ^[5-6].

Figure 1. TFR1 Structure ^[2]

1.2 TFR1 Expression

TFR1, a membrane protein expressed across various cell and tissue types in the human body, plays a crucial role in facilitating iron transport and metabolism. Its extensive presence includes immune system, hematopoietic system (e.g., bone marrow stem cells, red blood cells, and leukocytes), nervous system (e.g., neurons and glial cells), reproductive system, heart, liver, and kidney. TFR1 expression levels are susceptible to various factors such as intracellular iron concentration, cellular differentiation status, hormonal regulation, and inflammatory status ^[6-8].

1.3 TFR1 Function

TFR1's crucial physiological function is to bind with transferrin (Tf), thus facilitating the cellular uptake of iron by endocytosis. This Tf-TFR1 system is a vital pathway for the body to acquire iron ions. The process involves the binding of Tf and iron (Fe³⁺ or Fe²⁺) and a subsequent structural change that results in encapsulation of iron ions within the protein, forming Tf-Fe²⁺. At physiological pH, TFR1 binds to Tf-Fe²⁺, and the Tf-TFR1 complex undergoes clathrin-mediated endocytosis (Figure 2) ^[10-12].

The intracellular Tf-TFR1 complex then proceeds to the endosome for acidification, where amino acid residues interact, causing conformational changes that lead to the release of iron ions. TFR1 is recycled to the cell surface through the Golgi complex, completing the transport of iron ions. In summary, TFR1 plays an integral role in maintaining cellular and tissue functions. Regulating TFR1 to balance intracellular iron concentration and ensure iron homeostasis is essential for optimal physiological functioning ^[10-12].

Figure 2. Tf-TFR1 system balances intracellular iron concentration ^[2]

2. How's the Mechanism of Action of TFR1?

Abnormal or reduced TFR1 expression can lead to cellular iron deficiency, whereas excess iron may catalyze the production of reactive oxygen species (ROSs) and cause biomolecule damage. Cells have developed various mechanisms to regulate TFR1 expression to ensure adequate iron while avoiding its toxicity. However, the exact mechanisms and roles of aberrant TFR1 expression in disease are incompletely understood.

Different factors may affect the expression and function of TFR1 in cells. At the transcriptional level, hypoxia-inducible factor (HIF), c-Myc, GATA1, Ets-1, erythropoietin Stat5, and other transcription factors can regulate TFR1 expression. At the post-transcriptional level, IRP1 and IRP2 binding to IRE in TFR1 mRNA are key determinants ^[13-15].

Meanwhile, CD133 (PROM1) acts as a negative regulator. EGF receptor, c-Abl, and MARCH8 also play a role at the post-translational level. A study revealed that FLCN can control TFR1 expression, a protein involved in iron metabolism. This control occurs during or after protein synthesis by helping the Tf-TFR1 complex interact with Rab11-bound endosomes, which circulate within the cell and return to the cell membrane (Figure 3) ^[13-17].

TFR1 plays a role in regulating various diseases. In glioma, TFR1 affects multiple mechanisms related to inflammation, DNA, and cell cycle regulation. TFR1 also controls important signaling pathways such as PD1, IL17, IL18, NF-kβ, FOXM1, FOCAL, and JAK-STAT pathways. Besides, TFR1 knockdown in neural stem cells caused epilepsy symptoms in mice, with changes in synaptic GluA2 expression and neurotransmitter release ^[18-23].

Figure 3. FLCN regulates the recycling and transport of Tf-TFR1 protein via Rab11A ^[17]

3. The Roles of TFR1 in Tumor, Neurodegenerative and Other Diseases?

As a regulator of Fe transportation, TFR1 wields substantial influence over numerous health conditions. Specifically, TFRC plays a significant role in the development of anemia, neurodegenerative disorders, and a variety of cancers.

3.1 TFR1 and Tumors

Numerous studies have shown that TFR1 is abnormally expressed in various malignant tumors, including thyroid cancer ^[24], esophageal squamous cell carcinoma ^[25], breast cancer ^[26], liver cancer ^[27], colon cancer ^[28], leukemia ^[29], lung cancer ^[30], pancreatic cancer ^[31], and nasopharyngeal cancer ^[31]. However, there are certain malignancies, such as prostate cancer and testicular cancer, where the expression of TFR1 is not well-defined.

In hepatocellular carcinoma, the expression of TFR1 is correlated with the concentration of methemoglobin and serum thrombospondin ^[27]. In breast cancer, inhibiting IRP2 expression reduces TFR1 expression and increases ferritin heavy chain expression, which represses the growth of breast cancer cells ^{[26, 32]}.

In colon cancer, high expression of TFR1 activates the IL-6/IL-11-Stat3 signaling pathway, which promotes proliferation and apoptosis of colonic epithelial cells. This exacerbates colonic mucosa damage and accelerates the progression of colon cancer ^{[28, 33]}.

3.2 TFR1 and Neurodegenerative Diseases

Iron metabolism disturbances are known to be one of the pathophysiological mechanisms that trigger neurodegenerative diseases. Iron accumulation in the brain is associated with conditions such as Alzheimer's disease (AD), Parkinson's disease, and amyotrophic lateral sclerosis ^[34].

AD is one of the most common neurodegenerative diseases characterized by the accumulation of amyloid plaques and loss of certain neurons. Studies have shown that inhibiting the iron uptake proteins TFR1 and TF in the temporal cortex of Alzheimer's model mice, as well as reducing DMT1 expression, can effectively alleviate the iron overload state ^[35-36].

3.3 TFR1 and Anemia

TFR1 binding to the transferrin (Tf) complex is crucial for cells to acquire iron during erythropoiesis. When there is a shortage of iron or an increase in erythropoiesis, TFR1 expression is increased in response ^{[7, 37]}.

Clinical studies have confirmed that both expression levels of soluble transferrin receptor (sTFR1) and Tf are significantly higher than normal in thalassemic mice. Further studies have revealed that TFR1 is abnormally highly expressed in β-thalassemia precursor cells. Reducing TFR1 expression effectively regulates ineffective erythropoiesis, improving anemia and alleviating iron overload in affected mice ^{[7, 37]}.

3.4 TFR1 and Other Diseases

Studies suggest that TFR1 may also act as a viral receptor and participate in Hepatitis C Virus (HCV) cell entry by mediating HCV-host cell membrane fusion. Given that HCV is a major pathogen causing chronic hepatitis and primary liver cancer, the potential of TFR1 as an HCV antiviral target is subject to interest ^[38-40]. Furthermore, TFR1 has been discovered to regulate the transport of mGlu1 in neurons and is thought to participate in the mGlu1 signaling pathway, ultimately impacting motor coordination within the cerebellum ^[41-42].

4. Clinical Research Progress of TFR1

Currently, several clinical drugs targeting transferrin receptor 1 (TFR1) are under development, including antibody nucleic acid coupled drugs, bisomers, monomers, antibody fusion proteins, ADC drugs, etc.; the indications involve non-small-cell lung cancer, esophageal cancer, Mucopolysaccharidoses Storage Disease (MSD) type, muscular dystrophy, Alzheimer's Disease, and Duchenne Muscular Dystrophy. Among them, JCR Pharmaceuticals Co., Ltd.'sPabinafusp Alfa, an antibody fusion protein targeting IDS and TfR1, has been launched in Japan in 2021. TFR1-based targeted therapeutic strategies have been evolving in recent years. Some studies have utilized TFR1 to improve antibody transport across the blood-brain barrier and to combine with anti-β-amyloid peptide monoclonal antibodies to form specific complex antibodies to improve the therapeutic efficacy in patients with Alzheimer's disease ^[43-44].

Meanwhile, the anti-TFRC antibody JST-TFR09 and the anti-TFRC monoclonal antibody A24 inhibited iron uptake by tumor cells and induced apoptosis of malignant cells in T-lineage cell leukemia, respectively ^[45-46]. These studies suggest that TFR1 could be an effective target molecule involved in the clinical treatment of a variety of diseases. In the future, with the deepening and improvement of TFR1-related studies, it is expected to provide patients with more precise and effective treatments, bringing them more benefits and new hopes.

5. CUSABIO TFR1 related products

● CUSABIO TFR1 recombinant protein

Recombinant Human Transferrin receptor protein 1(TFRC),partial (Active) (Code: CSB-MP3648HU)

High Specificity Validated by SDS-PAGE

High specificity was validated by SDS-PAGE. SDS-PAGE (reduced) with 5% enrichment gel and 15% separation gel.

Excellent Bioactivity Validated by Functional ELISA

Immobilized Human TFRC at 2μg/mL can bind Anti-TFRC recombinant antibody (CSB-RA023441MA1HU), the EC₅₀ is 3.305-8.220 ng/mL.