Ferristatin II promotes degradation of transferrin receptor-1 in vitro and in vivo

Transferrin Receptor-Mediated Cellular Uptake of Fluorinated Chlorido[N,N'-bis(salicylidene)-1,2-phenylenediamine]iron(III) Complexes

Fluorinated chlorido[salophene]iron(III) complexes (salophene = N,N'-bis(salicylidene)-1,2-phenylenediamine) are promising anticancer agents. Apoptosis and necrosis induction have already been described as part of their mode of action. However, the involvement of ferroptosis in cell death induction, as confirmed for other chlorido[salophene]iron(III) complexes, has not yet been investigated. Furthermore, the mechanism of cellular uptake of these compounds is unknown. Therefore, the biological activity of the fluorescent chlorido[salophene]iron(III) complexes with a fluorine substituent at positions 3, 4, 5, or 6 at the salicylidene moieties (C1-C4) was evaluated in malignant and nonmalignant cell lines with focus on the involvement of the transferrin receptor-1 (TfR-1) in cellular uptake, the influence of the complexes on mitochondrial function, and the analysis of the molecular mechanism of cell death. All complexes significantly decreased the metabolic activity in the tested ovarian cancer (A2780, A2780cis), breast cancer (MDA-MB 231), and leukemia (HL-60) cell lines, while the nonmalignant human stroma cell line HS-5 at a concentration of 0.5 μM, which represents the IC50 of the complexes in most of the used tumorigenic cell lines, was not affected. The mitochondrial function was impaired, as evidenced by a reduced mitochondrial membrane potential ΔΨm and decreased mitochondrial activity. Besides apoptosis and necroptosis, ferroptosis was identified as part of the mode of action. It was further demonstrated for the first time that fluorinated chlorido[salophene]iron(III) complexes downregulate TfR-1 expression, comparable to ferristatin II, an iron transport inhibitor that acts via TfR-1 degradation. FerroOrange staining further indicated that the complexes strongly increased the intracellular iron(II) level as a driving force to induce ferroptosis. In conclusion, these fluorinated chlorido[salophene]iron(III) complexes are potent, tumor cell-specific chemotherapeutic agents, with the potential to treat various types of cancers.

The mechanism of mitochondrial autophagy regulating Clathrin-mediated endocytosis in epilepsy

OBJECTIVE

The objective of this study is to determine whether inhibition of mitophagy affects seizures through Clathrin-mediated endocytosis (CME).

METHODS

Pentylenetetrazol (PTZ) was intraperitoneally injected daily to establish a chronic PTZ-kindled seizure. The Western blot (WB) was used to compare the differences in Parkin protein expression between the epilepsy group and the control group. Immunofluorescence was used to detect the expression of MitoTracker and LysoTracker. Transferrin-Alexa488 (Tf-A488) was injected into the hippocampus of mice. We evaluated the effect of 3-methyladenine (3-MA) on epilepsy behavior through observation in PTZ-kindled models.

RESULTS

The methylated derivative of adenine, known as 3-MA, has been extensively utilized in the field of autophagy research. The transferrin protein is internalized from the extracellular environment into the intracellular space via the CME pathway. Tf-A488 uses a fluorescent marker to track CME. Western blot showed that the expression of Parkin was significantly increased in the PTZ-kindled model (p < 0.05), while 3-MA could reduce the expression (p < 0.05). The fluorescence uptake of MitoTracker and LysoTracker was increased in the primary cultured neurons induced by magnesium-free extracellular fluid (p < 0.05); the fluorescence uptake of Tf-A488 was significantly decreased in the 3-MA group compared with the control group (p < 0.05). Following hippocampal injection of Tf-A488, both the epilepsy group and the 3-MA group exhibited decreased fluorescence uptake, with a more pronounced effect observed in the 3-MA group. Inhibition of mitophagy by 3-MA from day 3 to day 9 progressively exacerbated seizure severity and shortened latency.

SIGNIFICANCE

It is speculated that the aggravation of seizures by 3-MA may be related to the failure to remove damaged mitochondria in time and effectively after inhibiting mitochondrial autophagy, affecting the vesicle endocytosis function of CME and increasing the susceptibility to epilepsy.

SUMMARY

Abnormal mitophagy was observed in a chronic pentylenetetrazol-induced seizure model and a Mg2+-free-induced spontaneous recurrent epileptiform discharge model. A fluorescent transferrin marker was utilized to track clathrin-mediated endocytosis. Using an autophagy inhibitor (3-methyladenine) on primary cultured neurons, we discovered that inhibition of autophagy led to a reduction in fluorescent transferrin uptake, while impairing clathrin-mediated endocytosis function mediated by mitophagy. Finally, we examined the effects of 3-methyladenine in an animal model of seizures showing that it exacerbated seizure severity. Ultimately, this study provides insights into potential mechanisms through which mitophagy regulates clathrin-mediated endocytosis in epilepsy.

HIF-2α/TFR1 mediated iron homeostasis disruption aggravates cartilage endplate degeneration through ferroptotic damage and mtDNA release: A new mechanism of intervertebral disc degeneration

Backgroud

Iron overload is a prevalent condition in the elderly, often associated with various degenerative diseases, including intervertebral disc degeneration (IDD). Nevertheless, the mechanisms responsible for iron ion accumulation in tissues and the mechanism that regulate iron homeostasis remain unclear. Transferrin receptor-1 (TFR1) serves as the primary cellular iron gate, playing a pivotal role in controlling intracellular iron levels, however its involvement in IDD pathogenesis and the underlying mechanism remains obscure.

Methods

Firstly, IDD mice model was established to determine the iron metabolism associated proteins changes during IDD progression. Then CEP chondrocytes were isolated and treated with TBHP or pro-inflammatory cytokines to mimic pathological environment, western blotting, immunofluorescence assay and tissue staining were employed to explore the underlying mechanisms. Lastly, TfR1 siRNA and Feristatin II were employed and the degeneration of IDD was examined using micro-CT and immunohistochemical analysis.

Results

We found that the IDD pathological environment, characterized by oxidative stress and pro-inflammatory cytokines, could enhance iron influx by upregulating TFR1 expression in a HIF-2α dependent manner. Excessive iron accumulation not only induces chondrocytes ferroptosis and exacerbates oxidative stress, but also triggers the innate immune response mediated by c-GAS/STING, by promoting mitochondrial damage and the release of mtDNA. The inhibition of STING through siRNA or the reduction of mtDNA replication using ethidium bromide alleviated the degeneration of CEP chondrocytes induced by iron overload.

Conclusion

Our study systemically explored the role of TFR1 mediated iron homeostasis in IDD and its underlying mechanisms, implying that targeting TFR1 to maintain balanced iron homeostasis could offer a promising therapeutic approach for IDD management.

The translational potential of this article

Our study demonstrated the close link between iron metabolism dysfunction and IDD, indicated that targeting TfR1 may be a novel therapeutic strategy for IDD.

TfR1 mediated iron metabolism dysfunction as a potential therapeutic target for osteoarthritis

OBJECTIVE

Transferrin receptor-1 (TfR1) plays important roles in controlling cellular iron levels, but its role in OA pathology is unknown. Herein we aim to investigate the role of TfR1 in OA progression and its underlying mechanisms.

METHODS

TfR1 expression in cartilage during OA development were examined both in vivo and in vitro. Then IL-1β was used to induce chondrocytes degeneration in vitro and TfR1 siRNA was used for observing the effect of TfR1 in modulating iron homeostasis, mitochondrial function and degrading enzymes expression. Also the inhibitor of TfR1 was exploited to analyze the protective effect of TfR1 inhibition in vivo.

RESULTS

TfR1 is elevated in OA cartilage and contributes to OA inflammation condition. Excess iron not only results in oxidative stress damage and sensitizes chondrocytes to ferroptosis, but also triggers c-GAS/STING-mediated inflammation by promoting mitochondrial destruction and the release of mtDNA. Silencing TfR1 using TfR1 siRNA not only reduced iron content in chondrocytes and inhibited oxidative stress, but also facilitated the mitophagy process and suppressed mtDNA/cGAS/STING-mediated inflammation. Importantly, we also found that Ferstatin II, a novel and selective TfR1 inhibitor, could substantially suppress TfR1 activity both in vivo and in vitro and ameliorated cartilage degeneration.

CONCLUSION

Our work demonstrates that TfR1 mediated iron influx plays important roles in chondrocytes degeneration and OA pathogenesis, suggesting that maintaining iron homeostasis through the targeting of TfR1 may represent a novel therapeutic strategy for the treatment of OA.

Influenza A virus exploits transferrin receptor recycling to enter host cells

Influenza A virus (IAV) enters host cells mostly through clathrin-dependent receptor-mediated endocytosis. A single bona fide entry receptor protein supporting this entry mechanism remains elusive. Here we performed proximity ligation of biotin to host cell surface proteins in the vicinity of attached trimeric hemagglutinin-HRP and characterized biotinylated targets using mass spectrometry. This approach identified transferrin receptor 1 (TfR1) as a candidate entry protein. Genetic gain-of-function and loss-of-function experiments, as well as in vitro and in vivo chemical inhibition, confirmed the functional involvement of TfR1 in IAV entry. Recycling deficient mutants of TfR1 do not support entry, indicating that TfR1 recycling is essential for this function. The binding of virions to TfR1 via sialic acids confirmed its role as a directly acting entry factor, but unexpectedly even headless TfR1 promoted IAV particle uptake in trans. TIRF microscopy localized the entering virus-like particles in the vicinity of TfR1. Our data identify TfR1 recycling as a revolving door mechanism exploited by IAV to enter host cells.

Investigating the trans-membrane transport of HAIYPRH peptide-decorated nano-drugs

Transferrin (Tf) has been effectively used to promote the cellular uptake of HAIYPRH (T7) peptide-conjugated nano-drugs. In this study, the enhancing effect of Tf on T7-decorated nano-drug transport was investigated using force tracing and nano-indentation techniques at a single-particle/cell level. Furthermore, the results were confirmed by ensemble fluorescence imaging.

Ferroptosis Signaling in Pancreatic β-Cells: Novel Insights & Therapeutic Targeting

Metabolic stress impairs pancreatic β-cell survival and function in diabetes. Although the pathophysiology of metabolic stress is complex, aberrant tissue damage and β-cell death are brought on by an imbalance in redox equilibrium due to insufficient levels of endogenous antioxidant expression in β-cells. The vulnerability of β-cells to oxidative damage caused by iron accumulation has been linked to contributory β-cell ferroptotic-like malfunction under diabetogenic settings. Here, we take into account recent findings on how iron metabolism contributes to the deregulation of the redox response in diabetic conditions as well as the ferroptotic-like malfunction in the pancreatic β-cells, which may offer insights for deciphering the pathomechanisms and formulating plans for the treatment or prevention of metabolic stress brought on by β-cell failure.

Molecular mimicry of the receptor-binding domain of the SARS-CoV-2 spike protein: from the interaction of spike-specific antibodies with transferrin and lactoferrin to the antiviral effects of human recombinant lactoferrin

The pathogenesis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection involves dysregulations of iron metabolism, and although the mechanism of this pathology is not yet fully understood, correction of iron metabolism pathways seems a promising pharmacological target. The previously observed effect of inhibiting SARS-CoV-2 infection by ferristatin II, an inducer of transferrin receptor 1 (TfR1) degradation, prompted the study of competition between Spike protein and TfR1 ligands, especially lactoferrin (Lf) and transferrin (Tf). We hypothesized molecular mimicry of Spike protein as cross-reactivity of Spike-specific antibodies with Tf and Lf. Thus, strong positive correlations (R² > 0.95) were found between the level of Spike-specific IgG antibodies present in serum samples of COVID-19-recovered and Sputnik V-vaccinated individuals and their Tf-binding activity assayed with peroxidase-labeled anti-Tf. In addition, we observed cross-reactivity of Lf-specific murine monoclonal antibody (mAb) towards the SARS-CoV-2 Spike protein. On the other hand, the interaction of mAbs produced to the receptor-binding domain (RBD) of the Spike protein with recombinant RBD protein was disrupted by Tf, Lf, soluble TfR1, anti-TfR1 aptamer, as well as by peptides RGD and GHAIYPRH. Furthermore, direct interaction of RBD protein with Lf, but not Tf, was observed, with affinity of binding estimated by K_D to be 23 nM and 16 nM for apo-Lf and holo-Lf, respectively. Treatment of Vero E6 cells with apo-Lf and holo-Lf (1–4 mg/mL) significantly inhibited SARS-CoV-2 replication of both Wuhan and Delta lineages. Protective effects of Lf on different arms of SARS-CoV-2-induced pathogenesis and possible consequences of cross-reactivity of Spike-specific antibodies are discussed.

Transferrin-mediated increase of labile iron Pool following simulated ischemia causes lipid peroxidation during the early phase of reperfusion

Heart ischemia/reperfusion (I/R) injury is related to iron content. However, the occurrence and mechanism of changes in labile iron pool (LIP) during I/R is debatable. Moreover, the identity of the iron form dominant in LIP during I/R is unclear. Herein, we measured changes of LIP during simulated ischemia (SI) and reperfusion (SR), in which ischemia was simulated in vitro with lactic acidosis and hypoxia. Total LIP did not change in lactic acidosis, whereas LIP, especially Fe³⁺, increased in hypoxia. Under SI, accompanied by hypoxia with acidosis, both Fe²⁺ and Fe³⁺ were significantly increased. Increased total LIP was maintained at 1 h post-SR. However, the Fe²⁺ and Fe³⁺ portion was changed. The increased Fe²⁺ was decreased, and conversely the Fe³⁺ was increased. BODIPY oxidized signal increased and through the time-course these changes correlated with blebbing of cell membrane and SR-induced LDH release. These data suggested lipid peroxidation occurred via Fenton's reaction. The experiments using bafilomycin A1 and zinc protoporphyrin suggested no role of ferritinophagy or heme oxidation in the increase of LIP during SI. The extracellular source, transferrin assessed using serum transferrin bound iron (TBI) saturation showed that the depletion of TBI reduced SR-induced cell damages and additive saturation of TBI accelerated SR-induced lipid peroxidation. Furthermore, Apo-Tf dramatically blocked the increase of LIP and SR-induced damages. In conclusion, Tf-mediated iron induces the increase of LIP during SI, and it causes Fenton reaction-mediated lipid peroxidation during the early phase of SR.

Angiotensin II type-1 receptor-associated protein interacts with transferrin receptor-1 and promotes its internalization

Kidney fibrosis is a common pathway that leads to chronic kidney disease. Angiotensin II type-1 receptor (AT1R)-associated protein (ATRAP) was originally identified as an AT1R-binding protein. Previously, we reported that systemic knockout of ATRAP exacerbates kidney fibrosis in aged mice. Although these effects of ATRAP appeared to be AT1R-independent actions, the molecular mechanism remains poorly understood. To elucidate the molecular mechanism of ATRAP independent of AT1R, we explored novel ATRAP-interacting proteins. Mass spectrometric analysis of the immunoprecipitants of a Flag-tagged ATRAP complex revealed 376 candidate proteins that potentially interact with ATRAP. Gene ontology analysis revealed that proteins related to vesicle trafficking, membrane transport, and many membrane proteins, including transferrin receptor 1 (TfR1), were enriched. Because TfR1 promotes cellular iron uptake and iron is a key factor involved in kidney fibrosis, we focused on TfR1 and confirmed that it interacts with ATRAP. In addition, our findings revealed that enhanced ATRAP expression decreased cell-surface TfR1 expression without altering the overall cellular TfR1 expression levels. Furthermore, enhanced ATRAP expression attenuated cellular iron levels. Together, our results highlight the role of ATRAP as a suppressor of TfR1 that functions by facilitating TfR1 internalization, which affects iron metabolism and oxidative stress signaling.

Mechanism of Ferroptosis and Its Relationships with Other Types of Programmed Cell Death: Insights for Potential Therapeutic Benefits in Traumatic Brain Injury

Traumatic brain injury (TBI) is a serious health issue with a high incidence, high morbidity, and high mortality that poses a large burden on society. Further understanding of the pathophysiology and cell death models induced by TBI may support targeted therapies for TBI patients. Ferroptosis, a model of programmed cell death first defined in 2012, is characterized by iron dyshomeostasis, lipid peroxidation, and glutathione (GSH) depletion. Ferroptosis is distinct from apoptosis, autophagy, pyroptosis, and necroptosis and has been shown to play a role in secondary brain injury and worsen long-term outcomes after TBI. This review systematically describes (1) the regulatory pathways of ferroptosis after TBI, (2) the neurobiological links between ferroptosis and other cell death models, and (3) potential therapies targeting ferroptosis for TBI patients.

Proteolytic Activity-Independent Activation of the Immune Response by Gingipains from Porphyromonas gingivalis

Porphyromonas gingivalis, a keystone pathogen in periodontitis (PD), produces cysteine proteases named gingipains (RgpA, RgpB, and Kgp), which strongly affect the host immune system. The range of action of gingipains is extended by their release as components of outer membrane vesicles, which efficiently diffuse into surrounding gingival tissues. However, away from the anaerobic environment of periodontal pockets, increased oxygen levels lead to oxidation of the catalytic cysteine residues of gingipains, inactivating their proteolytic activity. In this context, the influence of catalytically inactive gingipains on periodontal tissues is of significant interest. Here, we show that proteolytically inactive RgpA induced a proinflammatory response in both gingival keratinocytes and dendritic cells. Inactive RgpA is bound to the cell surface of gingival keratinocytes in the region of lipid rafts, and using affinity chromatography, we identified RgpA-interacting proteins, including epidermal growth factor receptor (EGFR). Next, we showed that EGFR interaction with inactive RgpA stimulated the expression of inflammatory cytokines. The response was mediated via the EGFR–phosphatidylinositol 3-kinase (PI3K)-protein kinase B (AKT) signaling pathway, which when activated in the gingival tissue rich in dendritic cells in the proximity of the alveolar bone, may significantly contribute to bone resorption and the progress of PD. Taken together, these findings broaden our understanding of the biological role of gingipains, which in acting as proinflammatory factors in the gingival tissue, create a favorable milieu for the growth of inflammophilic pathobionts.

Differences and Interactions in Placental Manganese and Iron Transfer across an In Vitro Model of Human Villous Trophoblasts

Manganese (Mn) as well as iron (Fe) are essential trace elements (TE) important for the maintenance of physiological functions including fetal development. However, in the case of Mn, evidence suggests that excess levels of intrauterine Mn are associated with adverse pregnancy outcomes. Although Mn is known to cross the placenta, the fundamentals of Mn transfer kinetics and mechanisms are largely unknown. Moreover, exposure to combinations of TEs should be considered in mechanistic transfer studies, in particular for TEs expected to share similar transfer pathways. Here, we performed a mechanistic in vitro study on the placental transfer of Mn across a BeWo b30 trophoblast layer. Our data revealed distinct differences in the placental transfer of Mn and Fe. While placental permeability to Fe showed a clear inverse dose-dependency, Mn transfer was largely independent of the applied doses. Concurrent exposure of Mn and Fe revealed transfer interactions of Fe and Mn, indicating that they share common transfer mechanisms. In general, mRNA and protein expression of discussed transporters like DMT1, TfR, or FPN were only marginally altered in BeWo cells despite the different exposure scenarios highlighting that Mn transfer across the trophoblast layer likely involves a combination of active and passive transport processes.

Ferristatin II, an Iron Uptake Inhibitor, Exerts Neuroprotection against Traumatic Brain Injury via Suppressing Ferroptosis

As a specific ferroptosis marker, transferrin receptor 1 (TfR1) expression is increased following traumatic brain injury (TBI), but the precise role of TfR1 in TBI-induced ferroptosis and neurodegeneration remains to be determined. To further identify more potent ferroptosis inhibitors and effective targets for treating TBI, our study aims at investigating the effects of TfR1 on ferroptosis in a mouse TBI model using ferristatin II (an iron uptake and TfR1 inhibitor). The effect of ferristatin II was first verified in the HT-22 cell line in vitro and showed antiferroptotic action when exposed to ferric citrate (FAC), which is in parallel with the results obtained from the positive controls, including deferoxamine (DFO) and liproxstatin-1 (Lip-1). In vivo, ferristatin II administration reduced the expression of TfR1 at 12 h after TBI, and immunofluorescence experiments further confirmed that this decreased TfR1-positive cells were neurons. Importantly, ferristatin II suppressed TBI-induced iron homeostatic imbalance by decreasing the content of Fe (III) and iron-positive deposits and reversed the expression of iron homeostasis-related proteins. Moreover, ferristatin II attenuated TBI-induced lipid peroxidation by reversing the expression of lipid peroxidative genes and proteins, as well as the increase in malondialdehyde (MDA) level following TBI. Finally, ferristatin II alleviated TBI-induced neuronal injury and neurodegeneration, as detected by staining with Nissl and Fluoro-Jade B, thereby exerting a neuroprotective effect. In summary, these data indicated that ferristatin II might be a potential strategy to restrain ferroptosis and develop novel therapeutic agents against TBI.

Ferristatin II Efficiently Inhibits SARS-CoV-2 Replication in Vero Cells

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to have a significant impact on global public health. Multiple mechanisms for SARS-CoV-2 cell entry have been described; however, the role of transferrin receptor 1 (TfR1) in SARS-CoV-2 infection has received little attention. We used ferristatin II to induce the degradation of TfR1 on the surface of Vero cells and to study the consequences of such treatment on the viability of the cells and the replication of SARS-CoV-2. We demonstrated that ferristatin II is non-toxic for Vero cells in concentrations up to 400 µM. According to confocal microscopy data, the distribution of the labeled transferrin and receptor-binding domain (RBD) of Spike protein is significantly affected by the 18h pretreatment with 100 µM ferristatin II in culture medium. The uptake of RBD protein is nearly fully inhibited by ferristatin II treatment, although this protein remains bound on the cell surface. The findings were well confirmed by the significant inhibition of the SARS-CoV-2 infection of Vero cells by ferristatin II with IC₅₀ values of 27 µM (for Wuhan D614G virus) and 40 µM (for Delta virus). A significant reduction in the infectious titer of the Omicron SARS-CoV-2 variant was noted at a ferristatin II concentration as low as 6.25 µM. We hypothesize that ferristatin II blocks the TfR1-mediated SARS-CoV-2 host cell entry; however, further studies are needed to elucidate the full mechanisms of this virus inhibition, including the effect of ferristatin II on other SARS-CoV-2 receptors, such as ACE2, Neuropilin-1 and CD147. The inhibition of viral entry by targeting the receptor on the host cells, rather than the viral mutation-prone protein, is a promising COVID-19 therapeutic strategy.

Iron disorders and hepcidin

Iron is an essential element due to its role in a wide variety of physiological processes. Iron homeostasis is crucial to prevent iron overload disorders as well as iron deficiency anemia. The liver synthesized peptide hormone hepcidin is a master regulator of systemic iron metabolism. Given its role in overall health, measurement of hepcidin can be used as a predictive marker in disease states. In addition, hepcidin-targeting drugs appear beneficial as therapeutic agents. This review emphasizes recent development on analytical techniques (immunochemical, mass spectrometry and biosensors) and therapeutic approaches (hepcidin agonists, stimulators and antagonists). These insights highlight hepcidin as a potential biomarker as well as an aid in the development of new drugs for iron disorders.

Evaluating the single-molecule interactions between targeted peptides and the receptors on living cell membrane

As potential ligands, targeted peptides have become an important part in the construction of intelligent drug delivery systems (DDSs). The targeting interaction of peptides with receptors is a key point affecting the efficacy of targeted nano-drugs. Herein, three common peptides (HAIYPRH (T7), YHWYGYTPQNVI (GE11), and RGD) that have been widely used in cancer targeted therapy and tumor diagnostics, targeting the corresponding receptors (transferrin receptor (TfR), epidermal growth factor receptor (EGFR), and α_νβ₃ integrin receptor), were selected as examples to study the targeting interacton on living cell surface at the single-molecule level by using single-molecule force spectroscopy (SMFS) based on atomic force microscopy (AFM). The dissociation activation energy in the absence of an external force (ΔG_β,0) of T7-TfR, GE11-EGFR, and RGD-α_νβ₃ integrin is evaluated at single-molecule level. Among these three peptide-receptor pairs, the T7-TfR bond is the most stable with a smaller dissociation kinetic rate constant at zero force (K_off), larger kinetic on-rate constant (K_on), and shorter interaction time (τ). Furthermore, T7 can target TfR even more effectively on A549 cell membrane after treatment with drugs. Our methodology can also be applicable to the study of other ligand targeted DDSs.

Transferrin-based radiolabeled probe predicts the sensitivity of human renal cancer cell lines to ferroptosis inducer erastin

Ferroptosis induction has been recognized as a novel cancer therapeutic strategy. To effectively apply ferroptosis-targeting cancer therapy to individual patients, a diagnostic indicator for selecting this therapeutic strategy from a number of molecular targeting drugs is needed. However, to date, methods that can predict the efficacy of ferroptosis-targeting treatment have not been established yet. In this study, we focused on the iron metabolic pathway to develop a nuclear imaging technique for diagnosing the susceptibility of cancer cells to ferroptosis. As a nuclear probe, human transferrin (Tf) was labeled with Gallium-68 (68Ga) using 2-(p-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) as a chelator (68Ga-NOTA-Tf). Western blot assay and clonogenic survival assay with human renal cancer cell lines A498 and 786-O revealed that the protein expression level of transferrin receptor1 (TfR1) and sensitivity to a ferroptosis inducer, erastin, were correlated. A cellular uptake assay with 68Ga-NOTA-Tf revealed that the cancer cells sensitive to erastin highly internalized the 68Ga-NOTA-Tf. Furthermore, treatment with the TfR1 inhibitor ferristatin II reduced the cellular uptake of 68Ga-NOTA-Tf, indicating that the intracellular uptake of the probe was mediated by TfR1. These results suggest that 68Ga-NOTA-Tf can be useful in predicting the sensitivity of cancer cells to ferroptosis inducers.

WDR45 Mutation Impairs the Autophagic Degradation of Transferrin Receptor and Promotes Ferroptosis

WDR45 is an autophagy-related protein that involves in the formation of autophagosome. Mutations in WDR45 lead to the impairment of autophagy which is associated with the human β-propeller protein-associated neurodegeneration (BPAN). However, the relationship between autophagy and brain iron accumulation in patients with BPAN remains unclear. Here, we demonstrated that transferrin receptor (TfRC) which is critical for the iron import of cells was degraded via autophagy. TfRC was accumulated after the inhibition of autophagy by treatment with autophagic inhibitor chloroquine or knockdown of ATG2A. The intracellular iron content was increased in cells overexpressing TfRC or mutant WDR45, however, ferritin H (FTH) chain was decreased. Increased TfRC and simultaneously decreased FTH consequently resulted in an elevated level of ferrous iron (Fe²⁺) which further promoted cell ferroptosis, demonstrated by the increased lipid peroxidation and reactive oxygen species (ROS) and the decreased glutathione peroxidase 4 (GPX4) and cell viability. Taken together, these findings provide a piece of important evidence that WDR45 deficiency impairs autophagic degradation of TfRC, therefore leading to iron accumulation, and the elevated iron promotes ferroptosis which may contribute to the progression of BPAN.

Revealing the Dynamic Mechanism by Which Transferrin Promotes the Cellular Uptake of HAIYPRH Peptide-Conjugated Nanostructures by Force Tracing

The HAIYPRH (T7) peptide has been widely used as a ligand for constructing tumor-targeted nanodrug delivery systems since it can target the transferrin receptor (TfR) and then enter cells easily with the help of transferrin (Tf). However, the dynamic mechanism by which transferrin promotes the entry of T7-conjugated nanostructures into cells remains unclear. Herein, a force tracing technique based on atomic force microscopy (AFM) was used to track the ultrafast dynamic process of a T7-conjugated gold nanoparticle (AuNP-T7) entering a cell at the single-particle level in real time. Tf helped decrease the endocytosis force and increase the endocytosis speed of AuNP-T7 in A549 cells. However, Tf only increased the endocytosis speed of AuNP-T7 in HeLa cells. In contrast, in Vero cells without TfR overexpression, Tf decreased the endocytosis speed. This report provides important insights for redesigning and developing T7-conjugated nanodrug carriers in targeted nanodrug delivery systems.

Transferrin receptor 1 levels at the cell surface influence the susceptibility of newborn piglets to PEDV infection

Porcine epidemic diarrhea virus (PEDV) mainly infects the intestinal epithelial cells of newborn piglets causing acute, severe atrophic enteritis. The underlying mechanisms of PEDV infection and the reasons why newborn piglets are more susceptible than older pigs remain incompletely understood. Iron deficiency is common in newborn piglets. Here we found that high levels of transferrin receptor 1 (TfR1) distributed in the apical tissue of the intestinal villi of newborns, and intracellular iron levels influence the susceptibility of newborn piglets to PEDV. We show that iron deficiency induced by deferoxamine (DFO, an iron chelating agent) promotes PEDV infection while iron accumulation induced by ferric ammonium citrate (FAC, an iron supplement) impairs PEDV infection in vitro and in vivo. Besides, PEDV infection was inhibited by occluding TfR1 with antibodies or decreasing TfR1 expression. Additionally, PEDV infection was increased in PEDV-resistant Caco-2 and HEK 293T cells over-expressed porcine TfR1. Mechanistically, the PEDV S1 protein interacts with the extracellular region of TfR1 during PEDV entry, promotes TfR1 re-localization and clustering, then activates TfR1 tyrosine phosphorylation mediated by Src kinase, and heightens the internalization of TfR1, thereby promoting PEDV entry. Taken together, these data suggest that the higher expression of TfR1 in the apical tissue of the intestinal villi caused by iron deficiency, accounts for newborn piglets being acutely susceptible to PEDV.

Newborn piglets are particularly susceptible to infection by PEDV, with 80–100% dying within days of infection. The reasons for newborns’ acute susceptibility to PEDV infection have not been elucidated clearly. The primarily target of PEDV is the porcine intestinal epithelial cells. Here, we show that the high expression of TfR1 in the apical tissue of intestinal villi in newborn piglets with iron deficiency is a reason for their susceptibility to PEDV. Further, we demonstrate that iron supplementation reduces PEDV infection. This study reveals that iron plays an important role in the susceptibility of newborn piglets to PEDV and provides insights into therapies for the prevention and treatment of PEDV infections.

Transferrin Receptor 1-Associated Iron Accumulation and Oxidative Stress Provides a Way for Grass Carp to Fight against Reovirus Infection

Iron is an essential element, closely linked with host immune responses. Nevertheless, the relationship between iron metabolism and virus infection is still unclear in aquatic vertebrates. To address this issue, we employed grass carp (Ctenopharyngodon idella) and its lethal virus, grass carp reovirus (GCRV), a double-strand RNA virus, as models. Our results demonstrate that GCRV infection increases the iron content and alters the expression of iron metabolism-related genes both in vivo and in vitro. Of note, the expression of C. idella transferrin receptor 1 (CiTfR1) rather than transferrin is upregulated upon GCRV infection. To clarify the implications of CiTfR1 upregulation for antiviral immunity, we proved that CiTfR1 was not a helper for GCRV invasion, but instead, it inhibited GCRV infection and promoted cell proliferation by facilitating the accumulation of intracellular labile iron pool (LIP), which increases intracellular oxidative stress. Interestingly, we found that CiTfR1 overexpression inhibited the mRNA expression of C. idella interferon 1 (CiIFN1) and CiIFN3. The present study reveals a novel antiviral defense mechanism in teleost where TfR1 induces the accumulation of LIP, leading to the suppression of virus infection and the proliferation of host cells, indicating that iron can be used as a medicated feed additive for the control of animal viral disease.

Crossing the Iron Gate: Why and How Transferrin Receptors Mediate Viral Entry

Because both the host and pathogen require iron, the innate immune response carefully orchestrates control over iron metabolism to limit its availability during times of infection. Nutritional iron deficiency can impair host immunity, while iron overload can cause oxidative stress to propagate harmful viral mutations. An emerging enigma is that many viruses use the primary gatekeeper of iron metabolism, the transferrin receptor, as a means to enter cells. Why and how this iron gate is a viral target for infection are the focus of this review.

Regulation of transferrin receptor-1 mRNA by the interplay between IRE-binding proteins and miR-7/miR-141 in the 3'-IRE stem-loops

Intracellular iron is tightly regulated by coordinated expression of iron transport and storage genes, such as transferrin receptor-1 (TfR1) and ferritin. They are primarily regulated by iron through iron-induced dissociation of iron-regulatory proteins (IRPs) from iron-responsive elements (IREs) in the 3'-UTR (untranslated region) of TfR1 or 5'-UTR of ferritin mRNA, resulting in destabilization of TfR1 mRNA and release of ferritin translation block. Thus high iron decreases iron transport via TfR1 mRNA degradation and increases iron storage via ferritin translational up-regulation. However, the molecular mechanism of TfR1 mRNA destabilization in response to iron remains elusive. Here, we demonstrate that miR-7-5p and miR-141-3p target 3'-TfR1 IREs and down-regulate TfR1 mRNA and protein expression. Conversely, miR-7-5p and miR-141-3p antagomiRs partially but significantly blocked iron- or IRP knockdown-induced down-regulation of TfR1 mRNA, suggesting the interplay between these microRNAs and IRPs along with involvement of another uncharacterized mechanism in TfR1 mRNA degradation. Luciferase reporter assays using 3'-UTR TfR1 IRE mutants suggested that the IREs C and E are targets of miR-7-5p and miR-141-3p, respectively. Furthermore, miR-7 expression was inversely correlated with TfR1 mRNA in human pancreatic adenocarcinoma patient samples. These results suggest a role of microRNAs in the TfR1 regulation in the IRP-IRE system.

Impaired Transferrin Receptor Palmitoylation and Recycling in Neurodegeneration with Brain Iron Accumulation

Neurodegeneration with brain iron accumulation (NBIA) is a genetically heterogeneous condition characterized by progressive dystonia with iron accumulation in the basal ganglia. How NBIA-associated mutations trigger iron overload remains poorly understood. After studying fibroblast cell lines from subjects carrying both known and unreported biallelic mutations in CRAT and REPS1, we ascribe iron overload to the abnormal recycling of transferrin receptor (TfR1) and the reduction of TfR1 palmitoylation in NBIA. Moreover, we describe palmitoylation as a hitherto unreported level of post-translational TfR1 regulation. A widely used antimalarial agent, artesunate, rescued abnormal TfR1 palmitoylation in cultured fibroblasts of NBIA subjects. These observations suggest therapeutic strategies aimed at targeting impaired TfR1 recycling and palmitoylation in NBIA.

Blocking transferrin receptor inhibits the growth of lung adenocarcinoma cells in vitro

Background

Transferrin receptor (TfR) is expressed in most lung cancers and is an indicator of poor prognosis in certain groups of patients. In this study, we blocked cell surface TfR to inhibit lung adenocarcinoma (LAC) cell growth in vitro and investigated the associated molecular mechanisms to determine a potential therapeutic target in human LAC.

Methods

RNA interference and antibody blocking techniques were used to block the function of TfR in LAC cells, and cell proliferation assays were used to detect the results. Affymetrix microarray analysis was conducted using H1299 cells in which TfR was blocked with an antibody to investigate the molecular mechanisms involved.

Results

The cell proliferation assay demonstrated that H1299 cell proliferation was significantly inhibited after small interfering RNA knockdown or blocking of TfR. Mechanistic studies found that 100 genes were altered more than two‐fold after TfR was blocked and that blocking TfR was accompanied by decreased expression of the oncogene KRAS.

Conclusion

Our data provide evidence that blocking TfR could significantly inhibit LAC proliferation by targeting the oncogene KRAS; therefore, TfR may be a therapeutic target for LAC. In addition, our results suggest a new method for blocking the signal from the oncogene KRAS by targeting TfR in LAC.

Targeting iron metabolism in drug discovery and delivery

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.

Developing drugs targeting transition metal homeostasis

Metal dyshomeostasis is involved in the pathogenesis and progression of diseases including cancer and neurodegenerative diseases. Metal chelators and ionophores are well known modulators of transition metal homeostasis, and a number of these molecules are in clinical trials. Metal-binding compounds are not the only drugs capable of targeting transition metal homeostasis. This review presents recent highlights in the development of chelators and ionophores for the treatment of cancer and neurodegenerative disease. Moreover, we discuss the development of small molecules that alter copper and iron homeostasis by inhibiting metal transport proteins. Finally, we consider the emergence of metal regulatory factor 1 as a drug target in diseases where it mediates zinc-induced signalling cascades leading to pathogenesis.

Hepcidin: A Promising Therapeutic Target for Iron Disorders: A Systematic Review

Iron is required for most forms of organisms, and it is the most essential element for the functions of many iron-containing proteins involved in oxygen transport, cellular respiration, DNA replication, and so on. Disorders of iron metabolism are associated with diverse diseases, including anemias (e.g., iron-deficiency anemia and anemia of chronic diseases) and iron overload diseases, such as hereditary hemochromatosis and β-thalassemia. Hepcidin (encoded by Hamp gene) is a peptide hormone synthesized by hepatocytes, and it plays an important role in regulating the systematic iron homeostasis. As the systemic iron regulator, hepcidin, not only controls dietary iron absorption and iron egress out of iron storage cells, but also induces iron redistribution in various organs. Deregulated hepcidin is often seen in a variety of iron-related diseases including anemias and iron overload disorders. In the case of iron overload disorders (e.g., hereditary hemochromatosis and β-thalassemia), hepatic hepcidin concentration is significantly reduced.Since hepcidin deregulation is responsible for iron disorder-associated diseases, the purpose of this review is to summarize the recent findings on therapeutics targeting hepcidin.Continuous efforts have been made to search for hepcidin mimics and chemical compounds that could be used to increase hepcidin level. Here, a literature search was conducted in PubMed, and research papers relevant to hepcidin regulation or hepcidin-centered therapeutic work were reviewed. On the basis of literature search, we recapitulated recent findings on therapeutic studies targeting hepcidin, including agonists and antagonists to modulate hepcidin expression or its downstream signaling. We also discussed the molecular mechanisms by which hepcidin level and iron metabolism are modulated.Elevating hepcidin concentration is an optimal strategy to ameliorate iron overload diseases, and also to relieve β-thalassemia phenotypes by improving ineffective erythropoiesis. Relative to the current conventional therapies, such as phlebotomy and blood transfusion, therapeutics targeting hepcidin would open a new avenue for treatment of iron-related diseases.

Role of hemoglobin and transferrin in multi-wall carbon nanotube-induced mesothelial injury and carcinogenesis

Multi‐wall carbon nanotubes (MWCNT) are a form of flexible fibrous nanomaterial with high electrical and thermal conductivity. However, 50‐nm MWCNT in diameter causes malignant mesothelioma (MM) in rodents and, thus, the International Agency of Research on Cancer has designated them as a possible human carcinogen. Little is known about the molecular mechanism through which MWCNT causes MM. To elucidate the carcinogenic mechanisms of MWCNT in mesothelial cells, we used a variety of lysates to comprehensively identify proteins specifically adsorbed on pristine MWCNT of different diameters (50 nm, NT50; 100 nm, NT100; 150 nm, NT150; and 15 nm/tangled, NTtngl) using mass spectrometry. We identified >400 proteins, which included hemoglobin, histone, transferrin and various proteins associated with oxidative stress, among which we selected hemoglobin and transferrin for coating MWCNT to further evaluate cytotoxicity, wound healing, intracellular catalytic ferrous iron and oxidative stress in rat peritoneal mesothelial cells (RPMC). Cytotoxicity to RPMC was observed with pristine NT50 but not with NTtngl. Coating NT50 with hemoglobin or transferrin significantly aggravated cytotoxicity to RPMC, with an increase in cellular catalytic ferrous iron and DNA damage also observed. Knockdown of transferrin receptor with ferristatin II decreased not only NT50 uptake but also cellular catalytic ferrous iron. Our results suggest that adsorption of hemoglobin and transferrin on the surface of NT50 play a role in causing mesothelial iron overload, contributing to oxidative damage and possibly subsequent carcinogenesis in mesothelial cells. Uptake of NT50 at least partially depends on transferrin receptor 1. Modifications of NT50 surface may decrease this human risk.

Regulators of Iron Homeostasis: New Players in Metabolism, Cell Death, and Disease

Iron is necessary for life, but can also cause cell death. Accordingly, cells evolved a robust, tightly regulated suite of genes for maintaining iron homeostasis. Previous mechanistic studies on iron homeostasis have granted insight into the role of iron in human health and disease. We highlight new regulators of iron metabolism, including iron-trafficking proteins [solute carrier family 39, SLC39, also known as ZRT/IRT-like protein, ZIP; and poly-(rC)-binding protein, PCBP] and a cargo receptor (NCOA4) that is crucial for release of ferritin-bound iron. We also discuss emerging roles of iron in apoptosis and a novel iron-dependent cell death pathway termed 'ferroptosis', the dysregulation of iron metabolism in human pathologies, and the use of iron chelators in cancer therapy.

The small molecule ferristatin II induces hepatic hepcidin expression in vivo and in vitro

Previous studies have shown that administration of ferristatin II to rats is associated with decreased serum iron, reduced transferrin saturation, and increased hepatic hepcidin expression. BMP and IL-6 signaling act via Smad and Stat3 transcription factors, respectively, to increase expression of hepcidin, the master regulator of iron metabolism. In this study, we aimed to explore the underlying mechanism of ferristatin II action on hepcidin production. We found that ferristatin II greatly increased hepcidin expression both in vivo and in vitro. In the rat liver, ferristatin II treatment decreased expression of Smad downstream targets Smad7 and Id1 and increased expression of Stat3 downstream targets α-2-macroglobulin, α-1-acid glycoprotein, and C-reactive peptide. Ferristatin II also increased Stat3 phosphorylation in the rat liver without affecting serum or hepatic IL-6 levels. It is unclear whether the Stat3 activation observed in vivo is a cause or a consequence to hepcidin induction. Reporter gene expression studies demonstrated that ferristatin II synergized with BMP6 and IL-6 to enhance hepcidin expression in vitro. However, this synergy was not due to activation of either Smad or Stat3 signaling, raising the possibility that ferristatin II may activate a novel pathway for hepcidin regulation.

Chaperone protein involved in transmembrane transport of iron

DMT1 (divalent metal transporter 1) is the main iron importer found in animals, and ferrous iron is taken up by cells via DMT1. Once ferrous iron reaches the cytosol, it is subjected to subcellular distribution and delivered to various sites where iron is required for a variety of biochemical reactions in the cell. Until now, the mechanism connecting the transporter and cytosolic distribution had not been clarified. In the present study, we have identified PCBP2 [poly(rC)-binding protein 2] as a DMT1-binding protein. The N-terminal cytoplasmic region of DMT1 is the binding domain for PCBP2. An interaction between DMT1 and PCBP1, which is known to be a paralogue of PCBP2, could not be demonstrated in vivo or in vitro. Iron uptake and subsequent ferritin expression were suppressed by either DMT1 or PCBP2 knockdown. Iron-associated DMT1 could interact with PCBP2 in vitro, whereas iron-chelated DMT1 could not. These results indicate that ferrous iron imported by DMT1 is transferred directly to PCBP2. Moreover, we demonstrated that PCBP2 could bind to ferroportin, which exports ferrous iron out of the cell. These findings suggest that PCBP2 can transfer ferrous iron from DMT1 to the appropriate intracellular sites or ferroportin and could function as an iron chaperone.