Intravesicular pH and iron uptake by immature erythroid cells

Pathophysiological aspects of transferrin-A potential nano-based drug delivery signaling molecule in therapeutic target for varied diseases

Transferrin (Tf), widely known for its role as an iron-binding protein, exemplifies multitasking in biological processes. The role of Tf in iron metabolism involves both the uptake of iron from Tf by various cells, as well as the endocytosis mediated by the complex of Tf and the transferrin receptor (TfR). The direct conjugation of the therapeutic compound and immunotoxin studies using Tf peptide or anti-Tf receptor antibodies as targeting moieties aims to prolong drug circulation time and augment efficient cellular drug uptake, diminish systemic toxicity, traverse the blood-brain barrier, restrict systemic exposure, overcome multidrug resistance, and enhance therapeutic efficacy with disease specificity. This review primarily discusses the various biological actions of Tf, as well as the development of Tf-targeted nano-based drug delivery systems. The goal is to establish the use of Tf as a disease-targeting component, accentuating the potential therapeutic applications of this protein.

Experimental and DFT Study of Monensinate and Salinomycinate Complexes Containing {Fe3(µ3-O)}7+ Core

Two trinuclear oxo-centred iron(III) coordination compounds of monensic and salinomycinic acids (HL) were synthesized and their spectral properties were studied using physicochemical/thermal methods (FT-IR, TG-DTA, TG-MS, EPR, Mössbauer spectroscopy, powder XRD) and elemental analysis. The data suggested the formation of [Fe3(µ3-O)L3(OH)4] and the probable complex structures were modelled using the DFT method. The computed spectral parameters of the optimized constructs were compared to the experimentally measured ones. In each complex, three metal centres were joined together at the axial position by a μ3-O unit to form a {Fe3O}7+ core. The antibiotics monoanions served as bidentate ligands through the carboxylate and hydroxyl groups located at the termini. The carboxylate moieties played a dual role bridging each two metal centres. Hydroxide anions secured the overall neutral character of the coordination species. Mössbauer spectra displayed asymmetric quadrupole doublets that were consistent with the existence of two types of high-spin iron(III) sites with different environments-two Fe[O5] and one Fe[O6] centres. The solid-state EPR studies confirmed the +3 oxidation state of iron with a total spin St = 5/2 per trinuclear cluster. The studied complexes are the first iron(III) coordination compounds of monensin and salinomycin reported so far.

An overview of ferroptosis in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is a public health problem associated with high mortality and high morbidity rates worldwide. Presently, its complex pathophysiology is still unclear, and there is no specific drug to reverse NAFLD. Ferroptosis is an iron-dependent and non-apoptotic form of cell death characterized by the iron-induced accumulation of lipid reactive oxygen species (ROS), which damage nucleic acids, proteins, and lipids; generate intracellular oxidative stress; and ultimately cause cell death. Emerging evidence indicates that ferroptosis is involved in the progression of NAFLD, although the mechanism of action of ferroptosis in NAFLD is still poorly understood. Herein, we summarize the mechanism of action of ferroptosis in certain diseases, especially in the pathogenesis of NAFLD, and discuss the potential therapeutic approaches currently used to treat NAFLD. This review also highlights further directions for the treatment and prevention of NAFLD and related diseases.

Effects of Antimalarial Drugs on Neuroinflammation-Potential Use for Treatment of COVID-19-Related Neurologic Complications

The SARS-CoV-2 virus that is the cause of coronavirus disease 2019 (COVID-19) affects not only peripheral organs such as the lungs and blood vessels, but also the central nervous system (CNS)—as seen by effects on smell, taste, seizures, stroke, neuropathological findings and possibly, loss of control of respiration resulting in silent hypoxemia. COVID-19 induces an inflammatory response and, in severe cases, a cytokine storm that can damage the CNS. Antimalarials have unique properties that distinguish them from other anti-inflammatory drugs. (A) They are very lipophilic, which enhances their ability to cross the blood-brain barrier (BBB). Hence, they have the potential to act not only in the periphery but also in the CNS, and could be a useful addition to our limited armamentarium against the SARS-CoV-2 virus. (B) They are non-selective inhibitors of phospholipase A₂ isoforms, including cytosolic phospholipase A₂ (cPLA₂). The latter is not only activated by cytokines but itself generates arachidonic acid, which is metabolized by cyclooxygenase (COX) to pro-inflammatory eicosanoids. Free radicals are produced in this process, which can lead to oxidative damage to the CNS. There are at least 4 ways that antimalarials could be useful in combating COVID-19. (1) They inhibit PLA_2. (2) They are basic molecules capable of affecting the pH of lysosomes and inhibiting the activity of lysosomal enzymes. (3) They may affect the expression and Fe²⁺/H⁺ symporter activity of iron transporters such as divalent metal transporter 1 (DMT1), hence reducing iron accumulation in tissues and iron-catalysed free radical formation. (4) They could affect viral replication. The latter may be related to their effect on inhibition of PLA₂ isoforms. Inhibition of cPLA₂ impairs an early step of coronavirus replication in cell culture. In addition, a secretory PLA₂ (sPLA₂) isoform, PLA2G2D, has been shown to be essential for the lethality of SARS-CoV in mice. It is important to take note of what ongoing clinical trials on chloroquine and hydroxychloroquine can eventually tell us about the use of antimalarials and other anti-inflammatory agents, not only for the treatment of COVID-19, but also for neurovascular disorders such as stroke and vascular dementia.

The metastasis suppressor NDRG1 down-regulates the epidermal growth factor receptor via a lysosomal mechanism by up-regulating mitogen-inducible gene 6

The metastasis suppressor, N-Myc downstream-regulated gene-1 (NDRG1) inhibits a plethora of oncogenic signaling pathways by down-regulating the epidermal growth factor receptor (EGFR). Herein, we examined the mechanism involved in NDRG1-mediated EGFR down-regulation. NDRG1 overexpression potently increased the levels of mitogen-inducible gene 6 (MIG6), which inhibits EGFR and facilitates its lysosomal processing and degradation. Conversely, silencing NDRG1 in multiple human cancer cell types decreased MIG6 expression, demonstrating the regulatory role of NDRG1. Further, NDRG1 overexpression facilitated MIG6-EGFR association in the cytoplasm, possibly explaining the significantly (p <0.001) increased half-life of MIG6 from 1.6 ± 0.2 h under control conditions to 7.9 ± 0.4 h after NDRG1 overexpression. The increased MIG6 levels enhanced EGFR co-localization with the late endosome/lysosomal marker, lysosomal-associated membrane protein 2 (LAMP2). An increase in EGFR levels after MIG6 silencing was particularly apparent when NDRG1 was overexpressed, suggesting a role for MIG6 in NDRG1-mediated down-regulation of EGFR. Silencing phosphatase and tensin homolog (PTEN), which facilitates early to late endosome maturation, decreased MIG6, and also increased EGFR levels in both the presence and absence of NDRG1 overexpression. These results suggest a role for PTEN in regulating MIG6 expression. Anti-tumor drugs of the di-2-pyridylketone thiosemicarbazone class that activate NDRG1 expression also potently increased MIG6 and induced its cytosolic co-localization with NDRG1. This was accompanied by a decrease in activated and total EGFR levels and its redistribution to late endosomes/lysosomes. In conclusion, NDRG1 promotes EGFR down-regulation through the EGFR inhibitor MIG6, which leads to late endosomal/lysosomal processing of EGFR.

New Perspectives on Iron Uptake in Eukaryotes

All eukaryotic organisms require iron to function. Malfunctions within iron homeostasis have a range of physiological consequences, and can lead to the development of pathological conditions that can result in an excess of non-transferrin bound iron (NTBI). Despite extensive understanding of iron homeostasis, the links between the “macroscopic” transport of iron across biological barriers (cellular membranes) and the chemistry of redox changes that drive these processes still needs elucidating. This review draws conclusions from the current literature, and describes some of the underlying biophysical and biochemical processes that occur in iron homeostasis. By first taking a broad view of iron uptake within the gut and subsequent delivery to tissues, in addition to describing the transferrin and non-transferrin mediated components of these processes, we provide a base of knowledge from which we further explore NTBI uptake. We provide concise up-to-date information of the transplasma electron transport systems (tPMETSs) involved within NTBI uptake, and highlight how these systems are not only involved within NTBI uptake for detoxification but also may play a role within the reduction of metabolic stress through regeneration of intracellular NAD(P)H/NAD(P)⁺ levels. Furthermore, we illuminate the thermodynamics that governs iron transport, namely the redox potential cascade and electrochemical behavior of key components of the electron transport systems that facilitate the movement of electrons across the plasma membrane to the extracellular compartment. We also take account of kinetic changes that occur to transport iron into the cell, namely membrane dipole change and their consequent effects within membrane structure that act to facilitate transport of ions.

Near-infrared fluorescent probes with higher quantum yields and neutral pKa values for the evaluation of intracellular pH

Near-infrared fluorescent probes for pH, named pH-A and pH-B, for labeling cells to produce high resolution fluorescent images reflect the changes of intracellular pH.

Accelerated neuronal differentiation toward motor neuron lineage from human embryonic stem cell line (H9)

Motor neurons loss plays a pivotal role in the pathoetiology of various debilitating diseases such as, but not limited to, amyotrophic lateral sclerosis, primary lateral sclerosis, progressive muscular atrophy, progressive bulbar palsy, pseudobulbar palsy, and spinal muscular atrophy. However, advancement in motor neuron replacement therapy has been significantly constrained by the difficulties in large-scale production at a cost-effective manner. Current methods to derive motor neuron heavily rely on biochemical stimulation, chemical biological screening, and complex physical cues. These existing methods are seriously challenged by extensive time requirements and poor yields. An innovative approach that overcomes prior hurdles and enhances the rate of successful motor neuron transplantation in patients is of critical demand. Iron, a trace element, is indispensable for the normal development and function of the central nervous system. Whether ferric ions promote neuronal differentiation and subsequently promote motor neuron lineage has never been considered. Here, we demonstrate that elevated iron concentration can drastically accelerate the differentiation of human embryonic stem cells (hESCs) toward motor neuron lineage potentially via a transferrin mediated pathway. HB9 expression in 500 nM iron-treated hESCs is approximately twofold higher than the control. Moreover, iron treatment generated more matured and functional motor neuron-like cells that are ∼1.5 times more sensitive to depolarization when compared to the control. Our methodology renders an expedited approach to harvest motor neuron-like cells for disease, traumatic injury regeneration, and drug screening.

Hepcidin and iron homeostasis during pregnancy

Hepcidin is the master regulator of systemic iron bioavailability in humans. This review examines primary research articles that assessed hepcidin during pregnancy and postpartum and report its relationship to maternal and infant iron status and birth outcomes; areas for future research are also discussed. A systematic search of the databases Medline and Cumulative Index to Nursing and Allied Health returned 16 primary research articles including 10 human and six animal studies. Collectively, the results indicate that hepcidin is lower during pregnancy than in a non-pregnant state, presumably to ensure greater iron bioavailability to the mother and fetus. Pregnant women with undetectable serum hepcidin transferred a greater quantity of maternally ingested iron to their fetus compared to women with detectable hepcidin, indicating that maternal hepcidin in part determines the iron bioavailability to the fetus. However, inflammatory states, including preeclampsia, malaria infection, and obesity were associated with higher hepcidin during pregnancy compared to healthy controls, suggesting that maternal and fetal iron bioavailability could be compromised in such conditions. Future studies should examine the relative contribution of maternal versus fetal hepcidin to the control of placental iron transfer as well as optimizing maternal and fetal iron bioavailability in pregnancies complicated by inflammation.

The regulation of cellular iron metabolism

While iron is an essential trace element required by nearly all living organisms, deficiencies or excesses can lead to pathological conditions such as iron deficiency anemia or hemochromatosis, respectively. A decade has passed since the discovery of the hemochromatosis gene, HFE, and our understanding of hereditary hemochromatosis (HH) and iron metabolism in health and a variety of diseases has progressed considerably. Although HFE-related hemochromatosis is the most widespread, other forms of HH have subsequently been identified. These forms are not attributed to mutations in the HFE gene but rather to mutations in genes involved in the transport, storage, and regulation of iron. This review is an overview of cellular iron metabolism and regulation, describing the function of key proteins involved in these processes, with particular emphasis on the liver's role in iron homeostasis, as it is the main target of iron deposition in pathological iron overload. Current knowledge on their roles in maintaining iron homeostasis and how their dysregulation leads to the pathogenesis of HH are discussed.

Iron chelation and regulation of the cell cycle: 2 mechanisms of posttranscriptional regulation of the universal cyclin-dependent kinase inhibitor p21CIP1/WAF1 by iron depletion

Iron (Fe) plays a critical role in proliferation, and Fe deficiency results in G(1)/S arrest and apoptosis. However, the precise role of Fe in cell-cycle control remains unclear. We observed that Fe depletion increased the mRNA of the universal cyclin-dependent kinase inhibitor, p21(CIP1/WAF1), while its protein level was not elevated. This observation is unique to the G(1)/S arrest seen after Fe deprivation, as increased p21(CIP1/WAF1) mRNA and protein are usually found when arrest is induced by other stimuli. In this study, we examined the posttranscriptional regulation of p21(CIP1/WAF1) after Fe depletion and demonstrated that its down-regulation was due to 2 mechanisms: (1) inhibited translocation of p21(CIP1/WAF1) mRNA from the nucleus to cytosolic translational machinery; and (2) induction of ubiquitin-independent proteasomal degradation. Iron chelation significantly (P < .01) decreased p21(CIP1/WAF1) protein half-life from 61 (+/- 4 minutes; n = 3) to 28 (+/- 9 minutes, n = 3). Proteasomal inhibitors rescued the chelator-mediated decrease in p21(CIP1/WAF1) protein, while lysosomotropic agents were not effective. In Fe-replete cells, p21(CIP1/WAF1) was degraded in an ubiquitin-dependent manner, while after Fe depletion, ubiquitin-independent proteasomal degradation occurred. These results are important for considering the mechanism of Fe depletion-mediated cell-cycle arrest and apoptosis and the efficacy of chelators as antitumor agents.

Transferrin: structure, function and potential therapeutic actions

There are many proteins that can multi-task. Transferrin, widely known as an iron-binding protein, is one such example of a multi-tasking protein. In this review, the multiple biological actions of transferrin, including its growth and cytoprotective activities, are discussed with the view of highlighting the potential therapeutic applications of this protein.

The transferrin homologue, melanotransferrin (p97), is rapidly catabolized by the liver of the rat and does not effectively donate iron to the brain

Melanotransferrin (MTf) or melanoma tumor antigen p97 is a membrane-bound transferrin (Tf) homologue that binds iron (Fe). This protein is also found as a soluble form in the plasma (sMTf) and was suggested to be an Alzheimer's disease marker. In addition, sMTf has been recently suggested to cross the blood-brain barrier (BBB) and accumulate in the brain of the mouse following intravenous infusion. Considering the importance of this observation to the physiology and pathophysiology of the BBB and the function of sMTf in vivo, we investigated the uptake and distribution of 59Fe-125I-sMTf and compared it to 59Fe-125I-Tf that were injected intravenously in rats. Studies were also performed to measure 59Fe and 125I-protein uptake by reticulocytes using these radiolabelled proteins. The results showed that sMTf was rapidly catabolized, mainly in the liver and to a lesser extent by the kidneys. The 59Fe was largely retained by these organs but the 125I was released into the plasma. Only a small amount of 125I-sMTf or its bound 59Fe was taken up by the brain, less than that from 59Fe-125I-Tf. There was much less 59Fe uptake by erythropoietic organs (spleen and femurs) from 59Fe-sMTf than from 59Fe-Tf, and no evidence of receptor-mediated uptake of sMTf was obtained using reticulocytes. It is concluded that compared to Tf, sMTf plays little or no role in Fe supply to the brain and erythropoietic tissue. However, a small amount of sMTf was taken up from the plasma by the brain and a far greater amount by the liver.

The soluble form of the membrane-bound transferrin homologue, melanotransferrin, inefficiently donates iron to cells via nonspecific internalization and degradation of the protein

Melanotransferrin (MTf) is a membrane-bound transferrin (Tf) homologue found particularly in melanoma cells. Apart from membrane-bound MTf, a soluble form of the molecule (sMTf) has been identified in vitro[Food, M.R., Rothenberger, S., Gabathuler, R., Haidl, I.D., Reid, G. & Jefferies, W.A. (1994) J. Biol. Chem.269, 3034-3040] and in vivo in Alzheimer's disease. However, nothing is known about the function of sMTf or its role in Fe uptake. In this study, sMTf labelled with 59Fe and 125I was used to examine its ability to donate 59Fe to SK-Mel-28 melanoma cells and other cell types. sMTf donated 59Fe to cells at 14% of the rate of Tf. Analysis of sMTf binding showed that unlike Tf, sMTf did not bind to a saturable Tf-binding site. Studies with Chinese hamster ovary cells with and without specific Tf receptors showed that unlike Tf, sMTf did not donate its 59Fe via these pathways. This was confirmed by experiments using lysosomotropic agents that markedly reduced 59Fe uptake from Tf, but had far less effect on 59Fe uptake from sMTf. In addition, an excess of 56Fe-labelled Tf or sMTf had no effect on 125I-labelled sMTf uptake, suggesting a nonspecific interaction of sMTf with cells. Protein-free 125I determinations demonstrated that in contrast with Tf, sMTf was markedly degraded. We suggest that unlike the binding of Tf to specific receptors, sMTf was donating Fe to cells via an inefficient mechanism involving nonspecific internalization and subsequent degradation.

Iron metabolism: iron deficiency and iron overload

Iron is an essential cofactor in a variety of cellular processes. Except for a few unusual bacterial species, iron is indispensable for living organisms. However, free iron is toxic because of its propensity to induce the formation of dangerous free radicals. Consequently, iron balance is tightly regulated. Disorders of iron homeostasis are among the most common afflictions of humans. This review discusses inherited iron deficiency and iron overload disorders and recent insights into their pathophysiology.

Non-transferrin-bound iron uptake in Belgrade and normal rat erythroid cells

Belgrade (b) rats have an autosomal recessive, microcytic, hypochromic anemia. Transferrin (Tf)-dependent iron uptake is defective because of a mutation in DMT1 (Nramp2), blocking endosomal iron efflux. This experiment of nature permits the present study to address whether the mutation also affects non-Tf-bound iron (NTBI) uptake and to use NTBI uptake compared to Tf-Fe utilization to increase understanding of the phenotype of the b mutation. The distribution of 59Fe2+ into intact erythroid cells and cytosolic, stromal, heme, and nonheme fractions was different after NTBI uptake vs. Tf-Fe uptake, with the former exhibiting less iron into heme but more into stromal and nonheme fractions. Both reticulocytes and erythrocytes exhibit NTBI uptake. Only reticulocytes had heme incorporation after NTBI uptake. Properly normalized, incorporation into b/b heme was approximately 20% of +/b, a decrease similar to that for Tf-Fe utilization. NTBI uptake into heme was inhibited by bafilomycin A1, concanamycin, NH4Cl, or chloroquine, consistent with the endosomal location of the transporter; cellular uptake was uninhibited. NTBI uptake was unaffected after removal of Tf receptors by Pronase or depletion of endogenous Tf. Concentration dependence revealed that NTBI uptake into cells, cytosol, stroma, and the nonheme fraction had an apparent low affinity for iron; heme incorporation behaved like a high-affinity process, as did an expression assay for DMT1. DMT1 serves in both apparent high-affinity NTBI membrane transport and the exit of iron from the endosome during Tf delivery of iron in rat reticulocytes; the low-affinity membrane transporter, however, exhibits little dependence on DMT1.

Primary cell cultures from murine kidney and heart differ in endosomal pH

Endosomal and lysosomal pH values have been determined for many established cultured cell lines of different origins. These cell lines may be grouped into two classes based on observed differences in pH of early (recycling) endosomes. Members of the first class typically have an early endosomal pH of 6.2, whereas members of the second class typically have an early endosomal pH of 5.4. Because established cell lines may have developed artificial differences in endosomal pH due to extended culture, it remains to be determined if endosomal pH differences exist in vivo and whether they are functionally significant. To address this question, we generated adherent primary explants from mouse kidney (primarily epithelial cells) and heart (primarily fibroblasts and cardiac muscle cells). Interestingly, enhanced acidification was observed in heart cell endosomes (pH = 5.5) compared with kidney cell endosomes (pH = 6.0). These results indicate that differences in endosomal pH do not solely arise from extended cell culture and imply that such differences may be important for the proper functioning of different cell types.

Cleavage of the transferrin receptor by human granulocytes: differential proteolysis of the exosome-bound TFR

In contrast with other mammalian granulocytes, human granulocytes rapidly cleave the transferrin receptor (TFR) from sheep exosomes. Proteolysis of TFR from exosomes is more rapid and more extensive than that from the sheep reticulocyte cell surface itself, although little difference in cleavage is seen when immunoprecipitates or when immobilized, solubilized receptors from the two sources are compared. Circulating exosomes but not the plasma membrane fraction from seven species of immature red cells or erythropoietic cells show the presence of a peptide of approximately 18 kD recognized by an antibody to the cytoplasmic domain of the TFR. This 18 kD peptide is virtually absent from the corresponding cellular plasma membranes including human reticulocyte membranes. Taken together, the data are consistent with the conclusion that the exosomes released to the circulation from maturing red cells are the principal source of the soluble, circulating, truncated TFR. The granulocyte protease appears to be present on the cell surface and not released into the medium after short (30 min) periods of incubation at 37 degrees C. The protease is inhibited by PMSF but only at high (1 mM) concentrations. Using sheep exosome bound-TFR as substrate, human granulocytes exceed other granulocytes in their capacity to cleave TFR, suggesting that this may be a key factor for the prominent amount of circulating, soluble receptor found in human sera during periods of elevated reticulocyte levels. Human exosomes, unlike those from other species, contain little native size TFR. Truncated receptor containing the cytoplasmic domain being predominant in human exosomes.

Use of inhibitors of ion transport to differentiate iron transporters in erythroid cells

Iron uptake by rabbit reticulocytes and mature erythrocytes was investigated using 4 incubation systems: 1. Fe-transferrin in NaCl at pH 7.4, 2. Fe-transferrin in sucrose at pH 5.9, 3. Fe(II)-sucrose in sucrose at pH 6.5, and 4.Fe(II)-sucrose in KCl at pH 7.0. These systems were compared with respect to their magnitude and response to many membrane transport inhibitors and modifying agents. Iron uptake via the first 3 systems had many similar features that were quite distinct from those of iron uptake in the fourth system. On the basis of these results, it is concluded that erythroid cells contain two iron transport mechanisms, one with high affinity and relatively low capacity for iron transport, which can be studied using incubation systems 1-3, and the other of low affinity but high capacity (incubation system 4). High-affinity transport is present only in immature erythroid cells, is relatively sensitive to inhibition by N-ethylmaleimide (NEM), N,N1- dicyclohexylcarbodiimide (DCCD), and 7-chloro-4-nitrobenz-2-oxa-1,3 diazole (NBD), and is probably the mechanism by which iron, released from transferrin within endosomes, is transported across the endosomal membrane into the cytosol. DCCD and NBD are also inhibitors of the endosomal H(+)-ATPase, which is in keeping with the hypothesis that this ATPase functions as the iron transporter in endosomal membranes. However, the more-specific inhibitor of this enzyme, bafilomycin A1, inhibited iron uptake only in incubation system 1, where its action can be attributed to inhibition of endosomal acidification. Hence, it is unlikely that the ATPase also functions as the iron transporter. The low-affinity uptake mechanism is sensitive to inhibition by amiloride, valinomycin, quinidine, imipramine, quercetin, and diethylstilbestrol (to all of which high-affinity transport is relatively resistant), and is present in mature erythrocytes as well as reticulocytes.

Effects of overexpression of the transferrin receptor on the rates of transferrin recycling and uptake of non-transferrin-bound iron

The possibilities that the recycling of the transferrin receptor is a rate-limiting step in the efflux of endocytosed transferrin, and that the receptor functions as a trans-membrane Fe transporter were investigated in untransfected Ltk- cells and in cells transfected with different levels of DNA for wild-type, mutant and chimeric human transferrin receptors. The uptake of transferrin-bound Fe and non-transferrin-bound Fe(II), and the surface binding, endocytosis and recycling of transferrin were measured. In cells that expressed increasing numbers of surface transferrin receptors, the rate of Fe uptake increased at a slower rate than the number of receptors. By measurement of the rates of endocytosis and recycling of transferrin it was shown that this effect was not due to a deficiency of endocytosis, but to a slower rate of recycling as the receptor numbers increased. Hence, a restricted recycling rate of the transferrin receptor appeared to be responsible for the slower rate of Fe uptake by cells with high receptor numbers, presumably because one or more cytosolic components required for recycling were in limited supply. The rate of uptake of non-transferrin-bound Fe(II) was not influenced by the number of transferrin receptors present on the surface of the cells even though this varied more than 20-fold between the different cell lines. Hence, this investigation does not support the hypothesis that the receptors play a direct role in the transport of Fe(II) across cell membranes, as has been proposed previously [Singer, S. J. (1989) Biol. Cell 65, 1-5].

Mechanisms of iron uptake by mammalian cells

On the basis of the discussion in this paper, the process of transferrin and iron (transferrin-bound iron and non transferrin-bound iron) uptake and transferrin release by reticulocytes are summarized diagrammatically. Although we were able to outline the pathways shown in the figure, there is still a long way to go before we achieve total understanding of the mechanisms of iron uptake. In addition, many important questions need to be answered. For example: what is the nature and properties of the iron carrier on the membrane? What is the relationship between the iron carrier and transferrin receptor? Is the iron carrier system in membranes of cells from different tissues similar or different? And how does iron cross the membrane of the endosomes after it is released from transferrin? All of these questions merit further investigation.

Two saturable mechanisms of iron uptake from transferrin in human melanoma cells: the effect of transferrin concentration, chelators, and metabolic probes on transferrin and iron uptake

The mechanisms of iron (Fe) and transferrin (Tf) uptake by the human melanoma cell line, SK-MEL-28, have been investigated using chelators and metabolic probes. These data provide evidence for two saturable processes of Fe uptake from Tf, namely, specific receptor-mediated endocytosis and a second nonspecific, non-receptor-mediated mechanisms which saturated with respect to Fe uptake at a Tf concentration of approximately 0.3 mg/ml. In contrast to Fe uptake, Tf uptake increased linearly up to at least 1 mg/ml. Furthermore, under the culture conditions used, the second nonspecific, non-receptor-mediated mechanism was the most important process in terms of quantitative Fe uptake. Two concentrations of Tf-125I-59Fe (0.01 and 0.1 mg/ml) were used in order to characterise the specific and nonspecific Fe uptake pathways. Membrane permeable chelators were equally effective at both Tf concentrations, whereas membrane impermeable chelators were significantly (P < 0.001) more effective at reducing the internalisation of Fe at the higher Tf concentration, consistent with a mechanism of Fe uptake which occurred at a site in contact with the extracellular medium. The oxidoreductase inhibitor, amiloride, only slightly inhibited Fe uptake at the higher Tf concentration, suggesting that the second nonspecific process was not mediated by a diferric Tf reductase. Three lysosomotrophic agents and the endocytosis inhibitor, phenylglyoxal, markedly reduced Fe uptake at both Tf concentrations, and it is concluded that a saturable process consistent with receptor-mediated endocytosis of Tf occurred at the lower Tf concentration, while the predominant mechanism of Fe uptake at high Tf concentrations was a second saturable process consistent with adsorptive pinocytosis.

Expression and loss of the transferrin receptor in growing and differentiating HD3 cells

During induced differentiation and maturation of HD3 cells (a chicken erythroblast cell line infected with a temperature-sensitive mutant of the avian erythroblastosis virus), the levels of transferrin receptor (TFR) and nucleoside transporter increase. Both these activities increase before elevated levels of hemoglobin are detected. Shortly after induction, as cellular TFR levels rise, a native-size TFR is detected in the cell-free culture medium, associated with an exosome fraction (100,000 xg pellet). Nucleoside transporter (measured as NBMPR-binding activity) is not increased in this pellet with induction. Previous studies have suggested that exosome formation in peripheral reticulocytes may be a significant route for loss of specific membrane proteins (Johnstone et al., 1991). Although the present experiments in HD3 cells do not address the quantitative importance of exosome formation, the studies suggest that exosome formation is an early event in commitment to the red cell lineage and is not a phenomenon restricted to the terminal stages of red cell maturation.

Endocytic vesicles contain a calmodulin-activated Ca2+ pump that mediates the inhibition of acidification by calcium

Endocytic vesicles isolated from rabbit reticulocytes contained an intrinsic Ca2(+)-pump activity that was dependent on ATP, activated by calmodulin and inhibited by vanadate. 45Ca2+ uptake and acidification studies indicated that acidification of the endocytic vesicles inversely correlated with the Ca2(+)-pump activity. Acidification was inhibited by externally added Ca2+ and calmodulin and activated by vanadate and EGTA. It is suggested that intravesicular Ca2+ can act as a modulator of endocytic vesicle acidification.

Plasma membrane Fe2-transferrin reductase and iron uptake in K562 cells are not directly related

Receptor-mediated endocytosis and recycling of transferrin is partly inhibited by the ferrous iron chelator bipyridine, which almost completely blocks iron uptake. Bipyridine causes iron release at the cell surface, but inhibition of iron uptake is also due to a blockage of its passage through the endosomal membrane. The rate of release of iron to bipyridine is decreased by the competing electron acceptor ferricyanide and by amiloride, but not by iron uptake inhibiting acidotropic amines. Transferrin reduction at the plasma membrane may be artificially induced by presence of a ferrous chelator and caused by low-affinity transmembrane NAD(P)H oxidoreductase.

Assay and characteristics of the iron binding moiety of reticulocyte endocytic vesicles

A 59Fe assay was designed to detect an Fe(III) binding capacity in NP-40 solubilized proteins from rabbit reticulocyte endocytic vesicles. The iron binding capacity had an apparent molecular weight as determined by gel exclusion chromatography of 450,000 daltons. The iron binding moiety coincided with the major nontransferrin iron-containing material of endocytic vesicles labeled in vivo by incubation of cells with 59Fe, 125I-labeled transferrin. The material solubilized from vesicles with NP-40 exhibited two classes of saturable binding sites, one with an association constant for 59Fe-citrate of 3.63 x 10(9) M-1 and with 6.6 x 10(-12) moles of iron bound per mg protein and the other with a constant of 3.96 x 10(8) M-1 and 1.0 x 10(-12) moles of iron bound per mg protein. These affinities are sufficient to satisfy the solubility characteristics of Fe(III) at pH 5.0. Most of the 59Fe bound both in vivo and in vitro to the iron binding moiety could be displaced with 56Fe and an equivalent amount of 59Fe could subsequently be rebound in vitro. The iron binding assay was adopted to vesicle proteins separated by SDS-polyacrylamide gel electrophoresis with subsequent transfer to nitrocellulose and revealed an iron binding activity of molecular weight approximately 95,000 daltons.

Transferrin endocytosis and iron uptake in developing myogenic cells in culture: effects of microtubular and metabolic inhibitors, sulphydryl reagents and lysosomotrophic agents

The experiments described in this study were designed to investigate receptor-mediated endocytosis of transferrin and its role in iron uptake by cultured chick presumptive myoblasts (dividing and non-dividing) and myotubes. The effects of a variety of inhibitors on the internalization of transferrin and iron were investigated and three main effects were found: (i) sulphydryl reagents and microtubular inhibitors reduced the rate of transferrin and iron internalization to similar degrees, (ii) metabolic inhibitors reduced the rate of iron uptake more than that of transferrin endocytosis, and (iii) lysosomotrophic agents almost completely abolished iron accumulation by the cells without any effect on the rate of transferrin internalization. The results suggest that metabolic energy is required not only for the endocytosis of transferrin but also for subsequent steps in the iron uptake process, and that iron release from transferrin occurs in acidified endosomes. Overall, these experiments show that all or virtually all of the iron taken up by developing muscle cells from transferrin occurs as a consequence of receptor-mediated endocytosis of the protein.

Effect of nicotine on transferrin binding and iron uptake by cultured rat placenta

The effect of nicotine on transferrin and iron transport in placental cells has been studied. Nicotine inhibits iron uptake but has little effect on the steady-state levels of transferrin. The effect is temperature and concentration dependent and is not reversible. At a concentration of 15 mM nicotine inhibited transferrin endocytosis by 40%, while iron uptake was decreased by nearly 60%. Nicotine exerted a similar effect on reticulocytes, but other amines, either tertiary or quaternary, had little or no effect on either iron uptake or steady-state intracellular transferrin levels. The results suggest that nicotine acts by blocking uptake, probably by acting as a weak base inhibiting iron release from transferrin, and inhibiting exocytosis with a resultant block of endocytosis. The concentrations required to exert an effect are too high to implicate inhibition of iron transport in the effects of smoking on pregnancy.

Effect of osmolar and ionic composition of the extracellular fluid on transferrin endocytosis and exocytosis and iron uptake by reticulocytes

The effects of osmolar and ionic factors on endocytosis and exocytosis were investigated using rabbit reticulocytes and 125I-59Fe labelled transferrin. Endocytosis and exocytosis of transferrin and the uptake of iron were inhibited by increasing the osmolality or decreasing the ionic strength or pH of the cell incubation medium. However, elevation of the pH above 8.0 inhibited endocytosis but not exocytosis. Replacement of the NaCl in the incubation medium by Nal, NaF, NaSCN, NaClO4, Na2SO4, Na phosphate, or Na Hepes inhibited endocytosis and iron uptake but only Nal, NaF, and NaSCN inhibited exocytosis. Transferrin exocytosis was insensitive to inhibitors of anion or cation transport, but endocytosis and iron uptake were inhibited by several anion transport inhibitors. Overall, transferrin endocytosis was more sensitive than exocytosis to most of the factors which were investigated, and the effects on the rates of endocytosis and iron uptake were quantitatively very similar. The results provide strong support for the concept that transferrin endocytosis is a necessary step in iron uptake by reticulocytes. They do not support the chemiosmotic models of exocytosis in their present formulations, but do not rule out the possible role of an osmotic event in exocytosis.

Uptake and subcellular processing of 59Fe-125I-labelled transferrin by rat liver

The uptake of transferrin and iron by the rat liver was studied after intravenous injection or perfusion in vitro with diferric rat transferrin labelled with 125I and 59Fe. It was shown by subcellular fractionation on sucrose density gradients that 125I-transferrin was predominantly associated with a low-density membrane fraction, of similar density to the Golgi-membrane marker galactosyltransferase. Electron-microscope autoradiography demonstrated that most of the 125I-transferrin was located in hepatocytes. The 59Fe had a bimodal distribution, with a larger peak at a similar low density to that of labelled transferrin and a smaller peak at higher density coincident with the mitochondrial enzyme succinate dehydrogenase. Approx. 50% of the 59Fe in the low-density peak was precipitated with anti-(rat ferritin) serum. Uptake of transferrin into the low-density fraction was rapid, reaching a maximal level after 5-10 min. When livers were perfused with various concentrations of transferrin the total uptakes of both iron and transferrin and incorporation into their subcellular fractions were curvilinear, increasing with transferrin concentrations up to at least 10 microM. Analysis of the transferrin-uptake data indicated the presence of specific transferrin receptors with an association constant of approx. 5 X 10(6) M-1, with some non-specific binding. Neither rat nor bovine serum albumin was taken up into the low-density fractions of the liver. Chase experiments with the perfused liver showed that most of the 125I-transferrin was rapidly released from the liver, predominantly in an undegraded form, as indicated by precipitation with trichloroacetic acid. Approx. 40% of the 59Fe was also released. It is concluded that the uptake of transferrin-bound iron by the liver of the rat results from endocytosis by hepatocytes of the iron-transferrin complex into low-density vesicles followed by release of iron from the transferrin and recycling of the transferrin to the extracellular medium. The iron is rapidly incorporated into mitochondria and cytosolic ferritin.

Fluorescence probe measurement of the pH of the transferrin microenvironment during iron uptake by rat bone marrow erythroid cells

Fluorescence probe measurements of the transferrin micro-environment during iron uptake by rat erythroid cells revealed that part of the transferrin is taken up in an acidic environment. The pH of this intracellular transferrin environment is 5.7. When rat erythroid cell precursors are incubated with diferric transferrin then in the incubation medium monoferric transferrins TfNFe and TfFeC appear. In view of the known instability of TfNFe at acidic pH, TfNFe cannot arise after endocytosis of Tf2Fe in acid vesicles at pH below 6.0. The results support the existence of a mechanism other than endocytosis in the iron uptake process in rat erythroid cells.

The mechanism of iron uptake by the rat placenta

The mechanism of iron uptake from transferrin by the rat placenta in culture has been studied. Transferrin endocytosis preceded iron accumulation by the cells. Both transferrin internalisation and iron uptake were inhibited by low temperature. Transferrin endocytosis was less susceptible to the effects of metabolic inhibitors such as sodium fluoroacetate, potassium cyanide, 2,4, dinitrophenol or carbonylcyanide M-chlorophenyl hydrazone (CCCP) than was iron uptake. Iron accumulation was decreased if the cells were incubated in the presence of weak bases such as chloroquine or ammonium chloride. These results suggest that, following internalisation, the vesicles containing the transferrin and iron became acidified, and that this acidification was a necessary prerequisite for the accumulation of iron by the cell. Further, the results indicate that the intravesicular pH was maintained at the expense of metabolic energy, suggesting that a pump may be involved. The importance of the permeability properties of the vesicle membrane in the iron uptake process was investigated by incubating the cells with labelled transferrin and iron in the presence of different cation and anion ionophores. Irrespective of the normal cation that the ionophores carried, all inhibited iron uptake without altering transferrin levels. In contrast, phloridzin, a Cl- transport inhibitor, did not affect either the levels of transferrin within the cells or the amount of iron accumulated.

Heme inhibits transferrin endocytosis in immature erythroid cells

The inhibitory effect of heme on iron uptake from transferrin by rat and rabbit reticulocytes and erythroid cells from the fetal rat liver was studied in vitro. Addition of hemin was shown to cause a decrease in the rate of transferrin endocytosis, the degree of inhibition being proportional to the reduction in iron uptake. The heme synthesis inhibitors, isoniazid and succinylacetone, stimulated the rate of transferrin endocytosis by 15-30% and caused a proportional increase in the rate of iron uptake, possibly by reducing the intracellular free heme concentration. It is concluded from these results that heme affects iron uptake by influencing the rate of transferrin endocytosis and recycling.