Cobalt and the Iron Acquisition Pathway: Competition towards Interaction with Receptor 1

Exploiting Co(III)-Cyclopentadienyl Complexes To Develop Anticancer Agents

In recent years, organometallic complexes have attracted much attention as anticancer therapeutics aiming at overcoming the limitations of platinum drugs that are currently marketed. Still, the development of half-sandwich organometallic cobalt complexes remains scarcely explored. Four new cobalt(III)-cyclopentadienyl complexes containing N,N-heteroaromatic bidentate, and phosphane ligands were synthesized and fully characterized by elemental analysis, spectroscopic techniques, and DFT methods. The cytotoxicity of all complexes was determined in vitro by the MTS assay in colorectal (HCT116), ovarian (A2780), and breast (MDA-MB-231 and MCF-7) human cancer cell lines and in a healthy human cell line (fibroblasts). The complexes showed high cytotoxicity in cancer cell lines, mostly due to ROS production, apoptosis, autophagy induction, and disruption of the mitochondrial membrane. Also, these complexes were shown to be nontoxic in vivo in an ex ovo chick embryo yolk sac membrane (YSM) assay.

Significance of conformation changes during the binding and release of chromium(III) from human serum transferrin

Trivalent chromium has been proposed to be transported in vivo from the bloodstream to the tissues via endocytosis by transferrin (Tf), the major iron transport protein in the blood. While both Cr(III) binding and release from Tf have been proposed to be too slow to be physiologically relevant, recent kinetic studies under physiological conditions demonstrate that Cr(III) binding and release are sufficiently fast to occur during the time of the endocytosis cycle (circa 15 min). Consequently, the release of Cr(III) from human and bovine serum Tf has been examined under conditions mimicking an endosome during endocytosis. These studies have also found that Cr(III)2-Tf can exist in multiple conformations giving rise to different spectroscopic properties and different rates of Cr(III) release. Time-dependent spectroscopic studies of the binding and release of Cr(III) from human serum Tf have been used to identify three different conformations of Cr(III)2-Tf. The conformation of Cr(III)2-Tf used in most previous studies forms too slowly to be physiologically relevant and slowly releases Cr(III) in endosomal pH range. The conformation formed between 5 min to 60 min after the addition of Cr(III) to apoTf at pH 7.4 in 25 mM bicarbonate resembles the conformation of Cr(III)2-Tf in its complex with Tf receptor (TfR) and loses Cr(III) rapidly at endosomal pH, although not as fast as the Tf-TfR complex. The significance of these conformations and the potential role of Tf in detoxification of Cr(III) are described.

Physical properties, ligand substitution reactions, and biological activity of Co(iii)-Schiff base complexes

Four cobalt(iii) complexes of the general formula [Co(Schiff base)(L)2]+, where L is ammonia (NH3) or 3-fluorobenzylamine (3F-BnNH2), were synthesized. The complexes were characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography. Their electrochemical properties, ligand substitution mechanisms, and ligand exchange rates in aqueous buffer were investigated. These physical properties were correlated to the cellular uptake and anticancer activities of the complexes. The complexes undergo sequential, dissociative ligand substitution, with the exchange rates depending heavily on the axial ligands. Eyring analyses revealed that the relative ligand exchange rates were largely impacted by differences in the entropy, rather than enthalpy, of activation for the complexes. Performing the substitution reactions in the presence of ascorbate led to a change in the reaction profile and kinetics, but no change in the final product. The cytotoxic activity of the complexes correlates with both the ligand exchange rate and reduction potential, with the more easily reduced and rapidly substituted complexes showing higher toxicity. These relationships may be valuable for the rational design of Co(iii) complexes as anticancer or antiviral prodrugs.

Cortical biometals: Changed levels in suicide and with mood disorders

BACKGROUND

Changes in levels of metals have been suggested to contribute to the pathophysiologies of several neurodegenerative disorders but to our knowledge this is the first metallomic study in CNS from patients with mood disorders. The focus of this study was on cortical regions affected by the pathophysiologies of bipolar disorders and major depressive disorders.

METHODS

Levels of metals were measured using inductively coupled plasma mass spectrometry in Brodmann's areas (BA) 6, 10 and 17 from patients with major depressive disorders (n = 13), bipolar disorders (n = 12) and age / sex matched controls (n = 13).

RESULTS

There were lower levels of cortical strontium (BA 6 & 10), ruthenium (BA 6 & 17) and cadmium (BA 10) from patients with major depressive disorder as well as lower levels of strontium in BA 10 from patients with bipolar disorders. Unexpectedly, there were changes in levels of 16 metals in the cortex, mainly BA 6, from suicide completers compared to those who died of other causes.

LIMITATIONS

Cohort sizes were relatively small but comparable with many studies using human postmortem CNS. Like all studies on non-treatment naïve patients, drug treatment was a potential confound in our experiments.

CONCLUSIONS

Our exploratory study suggests changes in levels of metals in bipolar disorders and major depressive disorders could be affecting cortical oxidative balance in patients with mood disorders. Our data raises the possibility that measuring levels of specific biometals in the blood could be used as a biomarker for increased risk of suicide.

Binding mechanism of uranyl to transferrin implicated by density functional theory study

The binding of uranyl to serum transferrin follows a Tyr* → Tyr* → Asp* stepwise mechanism.

Investigation of the roles of trace elements during hepatitis C virus infection using protein-protein interactions and a shortest path algorithm

BACKGROUND

Hepatitis is a type of infectious disease that induces inflammation of the liver without pinpointing a particular pathogen or pathogenesis. Type C hepatitis, as a type of hepatitis, has been reported to induce cirrhosis and hepatocellular carcinoma within a very short amount of time. It is a great threat to human health. Some studies have revealed that trace elements are associated with infection with and immune rejection against hepatitis C virus (HCV). However, the mechanism underlying this phenomenon is still unclear.

METHODS

In this study, we aimed to expand our knowledge of this phenomenon by designing a computational method to identify genes that may be related to both HCV and trace element metabolic processes. The searching procedure included three stages. First, a shortest path algorithm was applied to a large network, constructed by protein-protein interactions, to identify potential genes of interest. Second, a permutation test was executed to exclude false discoveries. Finally, some rules based on the betweenness and associations between candidate genes and HCV and trace elements were built to select core genes among the remaining genes.

RESULTS

12 lists of genes, corresponding to 12 types of trace elements, were obtained. These genes are deemed to be associated with HCV infection and trace elements metabolism.

CONCLUSIONS

The analyses indicate that some genes may be related to both HCV and trace element metabolic processes, further confirming the associations between HCV and trace elements. The method was further tested on another set of HCV genes, the results indicate that this method is quite robustness.

GENERAL SIGNIFICANCE

The newly found genes may partially reveal unknown mechanisms between HCV infection and trace element metabolism. This article is part of a Special Issue entitled "System Genetics" Guest Editor: Dr. Yudong Cai and Dr. Tao Huang.

Direct thermodynamic and kinetic measurements of Fe²⁺ and Zn²⁺ binding to human serum transferrin

Human serum transferrin (hTf) is a single-chain bilobal glycoprotein that efficiently delivers iron to mammalian cells by endocytosis via the transferrin/transferrin receptor system. While extensive studies have been directed towards the study of ferric ion binding to hTf, ferrous ion interactions with the protein have never been firmly investigated owing to the rapid oxidation of Fe(II) to Fe(III) and the difficulty in maintaining a fully anaerobic environment. Here, the binding of Fe(2+) and Zn(2+) ions to hTf has been studied under anaerobic and aerobic conditions, respectively, in the presence and absence of bicarbonate by means of isothermal titration calorimetry (ITC) and fluorescence spectroscopy. The ITC data indicate the presence of one class of strong binding sites with dissociation constants of 25.2 nM for Fe(2+) and 6.7 nM for Zn(2+) and maximum binding stoichiometries of 1 Zn(2+) (or 1 Fe(2+)) per hTf molecule. With either metal, the binding interaction was achieved by both favorable enthalpy and entropy changes (ΔH(0)~-12 kJ/mol and ΔS(0)~106 J/mol·K for Fe(2+) and ΔH(0)~-18 kJ/mol and ΔS(0)~97 J/mol·K for Zn(2+)). The large and positive entropy values are most likely due to the change in the hydration of the protein and the metal ions upon interaction. Rapid kinetics stopped-flow fluorescence spectroscopy revealed two different complexation mechanisms with different degrees of conformational changes upon metal ion binding. Our results are discussed in terms of a plausible scenario for iron dissociation from transferrin by which the highly stable Fe(3+)-hTf complex might be reduced to the more labile Fe(2+) ion before iron is released to the cytosol.

Transferrin receptor-1 iron-acquisition pathway - synthesis, kinetics, thermodynamics and rapid cellular internalization of a holotransferrin-maghemite nanoparticle construct

BACKGROUND

Targeting nanoobjects via the iron-acquisition pathway is always reported slower than the transferrin/receptor endocytosis. Is there a remedy?

METHODS

Maghemite superparamagnetic and theragnostic nanoparticles (diameter 8.6nm) were synthesized, coated with 3-aminopropyltriethoxysilane (NP) and coupled to four holotransferrin (TFe2) by amide bonds (TFe2-NP). The constructs were characterized by X-ray diffraction, transmission electron microscopy, FTIR, X-ray Electron Spectroscopy, Inductively Coupled Plasma with Atomic Emission Spectrometry. The in-vitro protein/protein interaction of TFe2-NP with transferrin receptor-1 (R1) and endocytosis in HeLa cells were investigated spectrophotometrically, by fast T-jump kinetics and confocal microscopy.

RESULTS

In-vitro, R1 interacts with TFe2-NP with an overall dissociation constant KD=11nM. This interaction occurs in two steps: in the first, the C-lobe of the TFe2-NP interacts with R1 in 50μs: second-order rate constant, k1=6×10(10)M(-1)s(-1); first-order rate constant, k-1=9×10(4)s(-1); dissociation constant, K1d=1.5μM. In the second step, the protein/protein adduct undergoes a slow (10,000s) change in conformation to reach equilibrium. This mechanism is identical to that occurring with the free TFe2. In HeLa cells, TFe2-NP is internalized in the cytosol in less than 15min.

CONCLUSION

This is the first time that a nanoparticle-transferrin construct is shown to interact with R1 and is internalized in time scales similar to those of the free holotransferrin.

GENERAL SIGNIFICANCE

TFe2-NP behaves as free TFe2 and constitutes a model for rapidly targeting theragnostic devices via the main iron-acquisition pathway.

Different binding affinities of Pb2+ and Cu2+ to glycosylation variants of human serum transferrin interfere with the detection of carbohydrate-deficient transferrin

Carbohydrate-deficient transferrin (CDT) is a specific biomarker of alcohol abuse, and for diagnosis of chronic alcohol, abuse is often determined using isoelectric focusing (IEF) and chromatographic techniques. To allow this method to be used for the diagnosis of alcohol abuse, inferences of various physical and chemical factors with the detection of CDT have been investigated. However, few reports have focused thus far on whether different metal ions have different binding affinities to CDT and HTf variants or further interfere in the detection of CDT. Here, in order to figure out whether and how metal ions such as Pb(2+) and Cu(2+) bind to holo-human serum transferrin (holo-HTf) and further interfere in CDT detection, the binding characteristics and the binding parameters of holo-HTf with metal ions such as Pb(2+) and Cu(2+) were investigated using UV-visible spectroscopy, Fluorescence spectroscopy, and ICP-MS. Moreover, whether the metal ions such as Pb(2+) and Cu(2+) will reduce the diagnostic accuracy of CDT in clinic was investigated using IEF. The present study demonstrates that Pb(2+) and Cu(2+) have different binding affinities to holo-HTf variants and produce different changes in the relative amounts of each glycosylation isoforms of HTf. Accordingly, the glycosylation chains of HTf will affect the binding affinities of glycosylation isoforms with Pb(2+) and Cu(2+), causing further interferences in CDT detection.

Uptake and release of metal ions by transferrin and interaction with receptor 1

BACKGROUND

For a metal to follow the iron acquisition pathway, four conditions are required: 1-complex formation with transferrin; 2-interaction with receptor 1; 3-metal release in the endosome; and 4-metal transport to cytosol.

SCOPE OF THE REVIEW

This review deals with the mechanisms of aluminum(III), cobalt(III), uranium(VI), gallium(III) and bismuth(III) uptake by transferrin and interaction with receptor 1.

MAJOR CONCLUSIONS

The interaction of the metal-loaded transferrin with receptor 1 takes place in one or two steps: a very fast first step (μs to ms) between the C-lobe and the helical domain of the receptor, and a second slow step (2-6h) between the N-lobe and the protease-like domain. In transferrin loaded with metals other than iron, the dissociation constants for the interaction of the C-lobe with TFR are in a comparable range of magnitudes 10 to 0.5μM, whereas those of the interaction of the N-lobe are several orders of magnitudes lower or not detected. Endocytosis occurs in minutes, which implies a possible internalization of the metal-loaded transferrin with only the C-lobe interacting with the receptor.

GENERAL SIGNIFICANCE

A competition with iron is possible and implies that metal internalization is more related to kinetics than thermodynamics. As for metal release in the endosome, it is faster than the recycling time of transferrin, which implies its possible liberation in the cell. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

The thermodynamic and binding properties of the transferrins as studied by isothermal titration calorimetry

BACKGROUND

In mammals, serum-transferrins transport iron from the neutral environment of the blood to the cytoplasm by receptor-mediated endocytosis. Extensive in-vitro studies have focused on the thermodynamics and kinetics of Fe(3+) binding to a number of transferrins. However, little attention has been given to the thermodynamic characterization of the interaction of transferrin with its receptor.

SCOPE OF REVIEW

Iron-loaded transferrin (Tf) binds with high affinity to the specific transferrin receptor (TfR) on the cell surface. The Tf-TfR complex is then internalized via receptor mediated endocytosis into an endosome where iron is released. Here, we provide an overview of recent studies that have used ITC to quantify the interaction of various metal ions with transferrin and highlight our current understanding of the thermodynamics of the transferrin-transferrin receptor system at physiological pH.

GENERAL SIGNIFICANCE

The interaction of the iron-loaded transferrin with the transferrin receptor is a key cellular process that occurs during the normal course of iron metabolism. Understanding the thermodynamics of this interaction is important for iron homeostasis since the physiological requirement of iron must be appropriately maintained to avoid iron-related diseases.

MAJOR CONCLUSIONS

The thermodynamic data revealed stoichiometric binding of all tested metal ions to transferrin with very high affinities ranging between 10(17) and 10(22)M(-1). Iron-loaded transferrin (monoferric or diferric) is shown to bind avidly (K~10(7)-10(8)M(-1)) to the receptor at neutral pH with a stoichiometry of one Tf molecule per TfR monomer. Significantly, both the N- and the C-lobe contribute to the binding interaction which is shown to be both enthalpically and entropically driven. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Absence of binding between the human transferrin receptor and the transferrin complex of biological toxic trace element, aluminum, because of an incomplete open/closed form of the complex

Human transferrin (Tf) very tightly binds two ferric ions to deliver iron to cells. Fe(III)(2)Tf (Fe(2)Tf) binds to the Tf receptor (TfR) at pH 7.4; however, iron-free Tf (apoTf) does not. Iron uptake is facilitated by endocytosis of the Fe(2)Tf-TfR complex. Tf can also bind aluminum ions, which cause toxic effects and are associated with many diseases. Since Al(III)(2)Tf (Al(2)Tf) does not bind to TfR, the uptake of aluminum by the cells does not occur through a TfR-mediated pathway. We have studied the absence of binding between Al(2)Tf and TfR by investigating the physicochemical characteristics of apoTf, Al(2)Tf, Fe(2)Tf, and TfR. The hydrodynamic radius of 38.8 A for Al(2)Tf obtained by dynamic light scattering was between that of 42.6 A for apoTf and 37.2 A for Fe(2)Tf. The zeta potential of -11.3 mV for Al(2)Tf measured by capillary electrophoresis was close to -11.2 mV for apoTf as compared to -11.9 mV for Fe(2)Tf, indicating that the Al(2)Tf surface had a relatively scarce negative charge as the apoTf surface had. These results demonstrated that the structure of Al(2)Tf was a trade-off between the closed and open forms of Fe(2)Tf and apoTf, respectively. Consequently, it is suggested that Al(2)Tf cannot form specific ionic interresidual interactions, such as those formed by Fe(2)Tf, to bind to TfR, resulting in impossible complex formation between Al(2)Tf and TfR.

The role of transferrin receptor 1 and 2 in transferrin-bound iron uptake in human hepatoma cells

Transferrin receptor (TFR) 1 and 2 are expressed in the liver; TFR1 levels are regulated by cellular iron levels while TFR2 levels are regulated by transferrin saturation. The aims of this study were to 1) determine the relative importance of TFR1 and TFR2 in transferrin-bound iron (TBI) uptake by HuH7 human hepatoma cells and 2) characterize the role of metal-transferrin complexes in the regulation of these receptors. TFR expression was altered by 1) incubation with metal-transferrin (Tf) complexes, 2) TFR1 and TFR2 small interfering RNA knockdown, and 3) transfection with a human TFR2 plasmid. TBI uptake was measured using (59)Fe-(125)I-labeled Tf and mRNA and protein expression by real-time PCR and Western blot analysis, respectively. Fe(2)Tf, Co(2)Tf, and Mn(2)Tf increased TFR2 protein expression, indicating that the upregulation was not specifically regulated by iron-transferrin but also other metal-transferrins. In addition, Co(2)Tf and Mn(2)Tf upregulated TFR1, reduced ferritin, and increased hypoxia-inducible factor-1alpha protein expression, suggesting that TFR1 upregulation was due to a combination of iron deficiency and chemical hypoxia. TBI uptake correlated with changes in TFR1 but not TFR2 expression. TFR1 knockdown reduced iron uptake by 80% while TFR2 knockdown did not affect uptake. At 5 microM transferrin, iron uptake was not affected by combined TFR1 and TFR2 knockdown. Transfection with a hTFR2 plasmid increased TFR2 protein expression, causing a 15-20% increase in iron uptake and ferritin levels. This shows for the first time that TFR-mediated TBI uptake is mediated primarily via TFR1 but not TFR2 and that a high-capacity TFR-independent pathway exists in hepatoma cells.

Kinetics and thermodynamics of metal-loaded transferrins: transferrin receptor 1 interactions

Transferrin receptor 1 (R) and human serum transferrin (T) are the two main actors in iron acquisition by the cell. R binds TFe2 (iron-loaded transferrin), which allows its internalization in the cytoplasm by endocytosis. T also forms complexes with metals other than iron. In order to follow the iron-acquisition pathway, these metals should obey at least two essential rules: (i) formation of a strong complex with T; and (ii) interaction of this complex with R. In the present paper, we propose a general mechanism for the interaction of five metal-loaded Ts [Fe(III), Al(III), Bi(III), Ga(III) and Co(III)] with R and we discuss their potential incorporation by the iron-acquisition pathway. With iron- and cobalt-loaded Ts, the interaction of R takes place in two steps: the first is detected by the T-jump technique and occurs in the 100 μs range, whereas the second is slow and occurs in the hour range. Bi(III)- and Ga(III)-loaded Ts interact with R in a single fast kinetic step, which occurs in the 100–500 μs range. No interaction is detected between R and aluminium-saturated T. The fast steps are ascribed to the interaction of the C-lobe of metal-loaded T with the helical domain of R: dissociation constant, K′1, of 0.50±0.07, 0.82±0.25, 4±0.4 and 1.10±0.12 μM for Fe(III), Co(III), Bi(III) and Ga(III) respectively. The second slow steps are ascribed to changes in the conformation of the protein–protein adducts which increase the stability to achieve, at thermodynamic equilibrium, an overall dissociation constant, K1, of 2.3 and 25 nM for Fe(III) and Co(III) respectively. This last step occurs over several hours, whereas endocytosis takes place in several minutes. This implies that metal-loaded Ts are internalized with only the C-lobe interacting with R. This suggests that, despite a lower affinity for R when compared with TFe2, some metal-loaded Ts can compete kinetically with TFe2 for the interaction with R and thus follow the iron-acquisition pathway.