Ti(IV) uptake and release by human serum transferrin and recognition of Ti(IV)-transferrin by cancer cells: understanding the mechanism of action of the anticancer drug titanocene dichloride

Enhancement of the cytotoxicity of titanocene dichloride by aging in organic co-solvent

Titanocene dichloride exhibits anti-proliferative activity in a wide spectrum of murine and human tumors. Although it is still unclear as to which species are active in biological media, they all readily deliver Ti(IV) to transferrin, the protein that transports iron in the blood. In this article, we report that aging of the complex in alcohols (namely methanol and ethanol) or dimethyl sulfoxide, the co-solvents used to prepare mother solution of the drug, leads to increased cytotoxic activity (i.e. lower IC(50) values) in HCT116 colon cancer cell lines, to a different extent. The TiCp(2)Cl(2) solvolysis was followed by (1)H NMR, ESI-MS, electrochemical and conductivity measurements, and the intracellular Ti(IV) uptake was evaluated.

A Mass Spectrometric and Molecular Modelling Study of Cisplatin Binding to Transferrin

A combination of mass spectrometry, UV/Vis spectroscopy and molecular modelling techniques have been used to characterise the interaction of cisplatin with human serum transferrin (Tf). Mass spectrometry indicates that cisplatin binds to the hydroxy functional group of threonine 457, which is located in the iron(III)-binding site on the C-terminal lobe of the protein. UV/Vis spectroscopy confirms the stoichiometry of binding and shows that cisplatin and iron(III) binding are competitive. The binding of cisplatin has been modelled by using molecular dynamic simulations and the results suggest that cisplatin can occupy part of both the iron(III)- and carbonate-binding sites in the C-terminal lobe of the protein. Combined, the studies suggest that cisplatin binding sterically restricts iron(III) binding to the C-terminal lobe binding site, whereas the N-terminal lobe binding site appears to be unaffected by the cisplatin interaction, possibly allowing the iron(III)-induced conformational change necessary for binding to a Tf receptor.

Intracellular mapping of the distribution of metals derived from the antitumor metallocenes

The intracellular distribution of transition metals in V79 Chinese hamster lung cells treated with subtoxic doses of the organometallic anticancer complexes Cp(2)MCl(2), where Cp is eta (5) -cyclopentadienyl and M is Mo, Nb, Ti, or V, has been studied by synchrotron-based X-ray fluorescence (XRF). While significantly higher concentrations of Mo and Nb were found in treated cells compared with control cells, distinct differences in the cellular distribution of each metal were observed. Analysis of thin sections of cells was consistent with some localization of Mo in the nucleus. Studies with a noncytotoxic thiol derivative of molybdocene dichloride showed an uneven distribution of Mo in the cells. For comparison, the low levels of Ti and V in cells treated with the more toxic titanocene and vanadocene complexes, respectively, resulted in metal concentrations at the detection limit of XRF. The results agree with independent chemical studies that have concluded that the biological chemistry of each of the metallocene dihalides is unique.

Organometallic chemistry, biology and medicine: ruthenium arene anticancer complexes

Our work has shown that certain ruthenium(II) arene complexes exhibit promising anticancer activity in vitro and in vivo. The complexes are stable and water-soluble, and their frameworks provide considerable scope for optimising the design, both in terms of their biological activity and for minimising side-effects by variations in the arene and the other coordinated ligands. Initial studies on amino acids and nucleotides suggest that kinetic and thermodynamic control over a wide spectrum of reactions of Ru(II) arene complexes with biomolecules can be achieved. These Ru(II) arene complexes appear to have an altered profile of biological activity in comparison with metal-based anticancer complexes currently in clinical use or on clinical trial.

Human serum transferrin cobalt complex: Stability and cellular uptake of cobalt

Transferrin (Tf) receptor expression is up-regulated on tumour cells. The human serum iron transport protein transferrin (Tf) can bind to many metals including gallium and cobalt. Cobalt has a positron-emitting isotope with a half-life of 18 h and would thus be a useful isotope for imaging purposes. This study has examined the stability of the Co-Tf in the presence of serum and albumin and the uptake of radioactive Co from Co-Tf by tumour cells. Dialysis of 57Co-Tf with serum or with apo-Tf resulted in loss of most 57Co from the complex. The time course of Co uptake from cells incubated with Co-Tf showed an initial rapid association with cells, then a slower rate of accumulation, that is, a similar uptake profile to that of iron. Competition and displacement experiments showed that uptake specifically occurred by interaction with Tf receptors.

Structural effects of titanium citrate on the human erythrocyte membrane

The structural effects of titanium citrate on the human erythrocyte membrane were studied through its interaction with intact erythrocytes and isolated unsealed human erythrocyte membranes (IUM). The studies were carried out by scanning electron microscopy and fluorescence spectroscopy, respectively. Titanium citrate induced shape changes in erythrocytes, which were damaged and ruptured leaving empty and retracted membranes. Fluorescence spectroscopy measurements in IUM indicated a disordering effect at both the polar head group and the acyl chain packing arrangements of the membrane phospholipid bilayer. Titanium citrate also interacted with molecular models of the erythrocyte membrane consisting in bilayers of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE), representing classes of phospholipids located in the outer and inner monolayers of the erythrocyte membrane, respectively. X-ray diffraction indicated that titanium citrate induced structural perturbation of the polar head group and of the hydrophobic acyl regions of DMPC, while the effects on DMPE bilayers were negligible. This conclusion is supported by fluorescence spectroscopy measurements on DMPC large unilamellar vesicles. All these findings indicate that the structural perturbations induced by titanium to human erythrocytes can be extended to other cells, thereby affecting their functions.

Production, processing and small animal PET imaging of titanium-45

INTRODUCTION

Titanium-45 was prepared as a tool for elucidation of the mechanism of action of titanium anticancer drugs in vivo using microPET imaging.

METHODS

Titanium-45 was produced by the 45Sc(p,n)45Ti nuclear reaction using 14.5 MeV protons. Sufficient yields of 45Ti were produced and separated from the target material with 99.8% radionuclidic purity using a simple, efficient separation procedure.

RESULTS

A typical bombardment of 5 microA for 1 h produced an average of 2105+/-150 MBq (56.9+/-4.0 mCi) at the end of bombardment (EOB), well within acceptable range of the calculated theoretical yields of 2165 MBq and 433 MBq microA-1 h-1 (58.5 mCi and 11.7 mCi microA-1 h-1). This amount of activity is sufficient for the radiosynthesis of target compounds as well as imaging studies. MicroPET images of a miniature Derenzo phantom show excellent resolution where rods of 1.25 mm were separated by four times their diameter.

CONCLUSIONS

Titanium-45 can be easily produced on a biomedical cyclotron with excellent yields as compared to calculated theoretical values with imaging studies demonstrating that the decay properties of titanium-45 are well suited for microPET.

Water-soluble titanocene complexes with sulfur-containing aminoacids: synthesis, spectroscopic, electrochemical and Ti(IV)–transferrin interaction studies

Three new water soluble titanocene-amino acid complexes have been synthesized via the reaction of Cp(2)TiCl(2) and two equivalents of amino acid (L) in methanol, affording [Cp(2)TiL(2)]Cl(2), L = L-cysteine (2), D-penicillamine (3) and L-methionine (4). These complexes have been characterized by (1)H, IR and UV-Vis spectroscopies, elemental analysis and cyclic voltammetry. Kinetic studies of ligand hydrolysis have been monitored at low pH using UV-Vis and (1)H NMR spectroscopies to assess their stability in aqueous solution. At low pH, amino acid ligands are lost one order of magnitude faster than cyclopentadienyl. However, at physiological pH, in Tris buffer solution, the complexes decompose rapidly to form an insoluble titanium compound. The affinity of these complexes to apo-transferrin was also investigated to elucidate how the ancillary amino acid ligands affect the titanium intake by apo-transferrin.

Double-strand hydrolysis of DNA by a magnesium(II) complex with diethylenetriamine

The development of artificial nucleases that hydrolyze DNA or RNA is of great interest in molecular biology, biotechnology, and medicine. We now report that a magnesium(II) complex of diethylenetriamine (Mg-dien) can effectively promote the double-stranded cleavage of plasmid DNA and the dideoxynucleotide dApdA under physiological conditions of pH and temperature. Experiments performed in the presence of hydrogen peroxide, radical scavengers, or under rigorously anaerobic conditions indicate that DNA cleavage mediated by Mg-dien occurs via a hydrolytic path. Mg-dien efficiently hydrolyzes supercoiled pBR322 DNA and the pseudo-first-order rate constant at 37 degrees C and pH 8.0 is estimated to be 1.60 h(-1). The dinucleotide dApdA hydrolysis, with Mg-dien at 170 microM, shows a rate enhancement factor of ca. 5 x 10(8). 1H and 31P(1H) NMR studies show that Mg-dien effectively hydrolyzes 5'-dAMP to give deoxyadenosine and inorganic phosphate. While Mg2+ has been found at the catalytic sites of many natural nucleases, Mg-dien appears to be the first synthetic Mg2+-containing system capable of hydrolyzing dideoxynucleotides and DNA and thus may provide a simple model system to assist mechanistic studies of naturally occurring nucleases.

Mechanisms of TfR-mediated transcytosis and sorting in epithelial cells and applications toward drug delivery

Transferrin receptor has been an important protein for many of the advances made in understanding the intricacies of the intramolecular sorting pathways of endocytosed molecules. The unique internalization and recycling functions of transferrin receptor have also made it an attractive choice for drug targeting and delivery of large protein-based therapeutics and toxins. Recent advances in elucidating the role of the intracellular controllers of transferrin recycling and sorting, such as Rab proteins and their effectors, have led to enhancement of transferrin receptor as a drug delivery vehicle. This review focuses on the use of transferrin receptor as an agent for facilitating drug delivery and targeting, and the role that mechanisms of transferrin receptor sorting and transcytosis play in these events.

A novel protein-mineral interface

Transferrins transport Fe3+ and other metal ions in mononuclear-binding sites. We present the first evidence that a member of the transferrin superfamily is able to recognize multi-nuclear oxo-metal clusters, small mineral fragments that are the most abundant forms of many metals in the environment. We show that the ferric ion-binding protein from Neisseria gonorrhoeae (nFbp) readily binds clusters of Fe3+, Ti4+, Zr4+ or Hf4+ in solution. The 1.7 A resolution crystal structure of Hf-nFbp reveals three distinct types of clusters in an open, positively charged cleft between two hinged protein domains. A di-tyrosyl cluster nucleation motif (Tyr195-Tyr196) is situated at the bottom of this cleft and binds either a trinuclear oxo-Hf cluster, which is capped by phosphate, or a pentanuclear cluster, which in turn can be capped with phosphate. This first high-resolution structure of a protein-mineral interface suggests a novel metal-uptake mechanism and provides a model for protein-mediated mineralization/dissimilation, which plays a critical role in geochemical processes.

Peptide-peptide interactions between human transferrin and transferrin-binding protein B from Moraxella catarrhalis

Transferrin-binding protein B (TbpB) is one component of a bipartite receptor in several gram-negative bacterial species that binds host transferrin and mediates the uptake of iron for growth. Transferrin and TbpB are both bilobed proteins, and the interaction between these proteins seems to involve similar lobe-lobe interactions. Synthetic overlapping peptide libraries representing the N lobe of TbpB from Moraxella catarrhalis were prepared and probed with labeled human transferrin. Transferrin-binding peptides were localized to six different regions of the TbpB N lobe, and reciprocal experiments identified six different regions of the C lobe of transferrin that bound TbpB. Truncations of the N lobe of TbpB that sequentially removed each transferrin-binding determinant were used to probe an overlapping peptide library of the C lobe of human transferrin. The removal of each TbpB N-lobe transferrin-binding determinant resulted in a loss of reactivity with peptides from the synthetic peptide library representing the C lobe of transferrin. Thus, individual peptide-peptide interactions between ligand and receptor were identified. A structural model of human transferrin was used to map surface regions capable of binding to TbpB.

Synergistic anion and metal binding to the ferric ion-binding protein from Neisseria gonorrhoeae

The 34-kDa periplasmic iron-transport protein (FBP) from Neisseria gonorrhoeae (nFBP) contains Fe(III) and (hydrogen)phosphate (synergistic anion). It has a characteristic ligand-to-metal charge-transfer absorption band at 481 nm. Phosphate can be displaced by (bi)carbonate to give Fe.CO(3).nFBP (lambda(max) 459 nm). The local structures of native Fe-PO(4)-nFBP and Fe.CO(3).nFBP were determined by EXAFS at the FeK edge using full multiple scattering analysis. The EXAFS analysis reveals that both phosphate and carbonate ligands bind to FBP in monodentate mode in contrast to transferrins, which bind carbonate in bidentate mode. The EXAFS analysis also suggests an alternative to the crystallographically determined position of the Glu ligand, and this in turn suggests that an H-bonding network may help to stabilize monodentate binding of the synergistic anion. The anions oxalate, pyrophosphate, and nitrilotriacetate also appear to serve as synergistic anions but not sulfate or perchlorate. The oxidation of Fe(II) in the presence of nFBP led to a weak Fe(III).nFBP complex (lambda(max) 471 nm). Iron and phosphate can be removed from FBP at low pH (pH 4.5) in the presence of a large excess of citrate. Apo-FBP is less soluble and less stable than Fe.nFBP and binds relatively weakly to Ga(III) and Bi(III) but not to Co(III) ions, all of which bind strongly to apo-human serum transferrin.

Study of the coordination of amino acids with metals using [trans-en2Os(eta2-H2)L]2+ as a 1H NMR probe

This is a study of the interaction of 23 kinds of amino acids, peptides and their analogues with Os((II)) at different pD values. Experiments show that in acidic conditions, the carboxyl group in amino acids can coordinate with Os((II)), and there exists H-D coupling of the dihydrogen of the probe with D2O in strongly acidic conditions, N does not coordinate with Os((II)); In alkaline conditions, the carboxyl group can coordinate with Os, and the coordinating species have trans and cis isomers, and the trans isomer can convert to cis with time; N of -NH2- in alpha-amino group can coordinate with Os((II)) while that in gamma-amino-n-butyric acid cannot do that. Since the target of some anti tumor agents are nucleic acids and proteins, we demonstrate a competitive mode to study how the anti tumor complex Me2SnCl2 binds to amino acid Ala, and the minimum binding amount and formation constant of the metal anti tumor metal complexes binding with amino acid are also obtained.

Titanium complexes in cancer treatment

A series of complexes containing titanium, Ti, as a metal center has shown to possess a wide spectrum of antitumor properties. This series belongs to the non-platinum metal antitumor agents which has been developed mainly in the past 20 years. The bis(beta-diketonato)titanium(IV) and titanocene derivatives appear to offer a different alternative for cancer chemotherapy which do not follow the rationale and mechanism of action of the platinum complexes. The hydrolysis of these complexes in aqueous and pseudo aqueous solutions is discussed and the interaction studies of titanium complexes with biomolecules are also presented to unravel the mechanism of action at molecular levels.

Transferrin/transferrin receptor-mediated drug delivery

Since transferrin was discovered more than half a century ago, a considerable effort has been made towards understanding tranferrin-mediated iron uptake. However, it was not until recently with the identification and characterization of several new genes related to iron homeostasis, such as the hemochromatosis protein HFE and the iron transporter DMT1, that our knowledge has been advanced dramatically. A major pathway for cellular iron uptake is through internalization of the complex of iron-bound transferrin and the transferrin receptor, which is negatively modulated by HFE, a protein related to hereditary hemochromatosis. Iron is released from transferrin as the result of the acidic pH in endosome and then is transported to the cytosol by DMT1. The iron is then utilized as a cofactor by heme and ribonucleotide reductase or stored in ferritin. Apart from iron, many other metal ions of therapeutic and diagnostic interests can also bind to transferrin at the iron sites and their transferrin complexes can be recognized by many cells. Therefore, transferrin has been thought as a "delivery system" for many beneficial and harmful metal ions into the cells. Transferrin has also be widely applied as a targeting ligand in the active targeting of anticancer agents, proteins, and genes to primary proliferating malignant cells that overexpress transferrin receptors. This is achieved by conjugation of transferrin with drugs, proteins, hybride systems with marcomolecules and as liposomal-coated systems. Conjugates of anticancer drugs with transferrin can significantly improve the selectivity and toxicity and overcome drug resistance, thereby leading to a better treatment. The coupling of DNA to transferrin via a polycation such as polylysine or via cationic liposomes can target and transfer of the extrogenous DNA particularly into proliferating cells through receptor-mediated endocytosis. These kinds of non-viral vectors are potential alternatives to viral vectors for gene therapy, if the transfection efficiency can be improved. Moreover, transferrin receptors have shown potentials in delivery of therapeutic drugs or genes into the brain across blood-brain barrier.

The role of the transferrin-transferrin-receptor system in drug delivery and targeting

Exploration of the potential of site-specific and target-oriented drug delivery systems has gained interest recently. Indeed, the efficient cellular mechanism of transferrin uptake has been exploited for the delivery not only of anticancer drugs and proteins, but also of therapeutic genes into proliferating malignant cells that overexpress transferrin receptors. In particular, the transferrin receptor offers great promise in the delivery of therapeutic agents across the blood-brain barrier to the brain.

Determination of drug binding sites to proteins by electrospray ionisation mass spectrometry: the interaction of cisplatin with transferrin

Cisplatin, cis-[PtCl2(NH3)2], is known to bind to human serum transferrin, but the binding site remains a matter of some debate. Electrospray ionisation mass spectrometry has been used to characterise the interaction of cisplatin with transferrin. The studies indicate that cisplatin initially docks with, and subsequently bonds covalently to, the hydroxyl functional group of threonine 457, with the loss of HCl affording a transferrin-O-PtCl(NH3)2 adduct.

Competitive binding of bismuth to transferrin and albumin in aqueous solution and in blood plasma

Several bismuth compounds are currently used as antiulcer drugs, but their mechanism of action is not well established. Proteins are thought to be target sites. In this work we establish that the competitive binding of Bi(3+) to the blood serum proteins albumin and transferrin, as isolated proteins and in blood plasma, can be monitored via observation of (1)H and (13)C NMR resonances of isotopically labeled [epsilon-(13)C]Met transferrin. We show that Met(132) in the I132M recombinant N-lobe transferrin mutant is a sensitive indicator of N-lobe metal binding. Bi(3+) binds to the specific Fe(3+) sites of transferrin and the observed shifts of Met resonances suggest that Bi(3+) induces similar conformational changes in the N-lobe of transferrin in aqueous solution and plasma. Bi(3+) binding to albumin is nonspecific and Cys(34) is not a major binding site, which is surprising because Bi(3+) has a high affinity for thiolate sulfur. This illustrates that the potential target sites for metals (in this case Bi(3+)) in proteins depend not only on their presence but also on their accessibility. Bi(3+) binds to transferrin in preference to albumin both in aqueous solution and in blood plasma.