Vanadium(V/IV)–Transferrin Binding Disrupts the Transferrin Cycle and Reduces Vanadium Uptake and Antiproliferative Activity in Human Lung Cancer Cells

Abstract

The role of vanadium binding to transferrin (Tf) in the biological activities of vanadium-based drugs is a matter of considerable debate. In order to determine whether V­(V) and/or V­(IV) binding to Tf (in apo, monoferric­(III), and diferric­(III) forms) enhances or inhibits biological activities, cellular V uptake and in vitro antiproliferative activity were examined in the presence and absence of different forms of Tf and other biomolecules under normoxic conditions. These data were combined with studies on V–Tf binding in cell culture medium and its role in Tf interactions with transferrin receptor 1 (TfR1), using the biolayer interferometry (BLI) model of the Tf cycle that was developed in our group. The results showed that both V­(V) and V­(IV) oxidation states efficiently bind to vacant Fe­(III) binding sites of Tf even in the presence of a 20-fold molar excess of albumin, although V does not displace Tf-bound Fe­(III) under these conditions. Binding of V­(V) or V­(IV) to Tf in cell culture medium drastically reduced its cellular uptake and antiproliferative activity in the A549 (human lung cancer) cell line that expresses TfR1. BLI and gel electrophoresis studies showed that V­(V/IV) binding to partially Fe­(III) saturated Tf did not enhance the affinity of Tf binding to TfR1 at pH 7.4 but did disrupt Tf conformational changes under endosome-mimicking conditions (pH 5.6, 0.10 mM citrate). Hence, it is postulated that the absence of a significant cellular uptake of Tf-bound V­(V/IV) is likely to be due to the return of undissociated V­(V/IV)–Tf adducts to the cell surface after the endosomal step. Collectively, these data show that the biotransformation of V-based drugs leads to V­(V/IV)–Tf binding in the blood serum and inhibits, rather than enhances, the biological activity of such drugs under aerobic conditions. These results indicate that the design of V-based drugs that are stable enough to survive in the blood, enter cells intact, and release the active components intracellularly is likely to be required for their clinical success.

A Short‐Lived but Highly Cytotoxic Vanadium(V) Complex as a Potential Drug Lead for Brain Cancer Treatment by Intratumoral Injections

“…Thus, contrary to literature postulates, [19] V binding to Tf is not required for its cellular uptake and cytotoxicity, as described elsewhere. [21] The higher cytotoxicity of fresh 1 vs. its decomposition products in all cell lines ( Figure 2) correlated with higher cellular V content ( Figure S4). These results support the postulate [11] that 1 is able to enter cells intact due to its hydrophobic nature, which leads to its increased stability and permeability in cell membranes.…”

Exploring Serum Transferrin Regulation of Nonferric Metal Therapeutic Function and Toxicity

“…In addition, Fe 2 -sTf could also play a significant transporter role with nonferric metal ions in chelate form adventitiously coordinating to surface sites of the protein. Chromium(III) (Cr(III)), ruthenium(II/III) (Ru(II/III)), and vanadium(IV) (V(IV)) complexes have been reported to bind to the surface of Fe 2 -sTf [64][65][66][67][68]. Fe 2 -sTf is maximally primed for TfR delivery and it is feasible that other metals could ride on its shoulders.…”

Exploring Serum Transferrin Regulation of Nonferric Metal Therapeutic Function and Toxicity

“…In addition, Fe2-sTf could also play a significant transporter role with nonferric metal ions in chelate form adventitiously coordinating to surface sites of the protein. Chromium(III) (Cr(III)), ruthenium(II/III) (Ru(II/III)), and vanadium(IV) (V(IV)) complexes have been reported to bind to the surface of Fe2-sTf [64][65][66][67][68]. Fe2-sTf is maximally primed for TfR delivery and it is feasible that other metals could ride on its shoulders.…”

“…The Urea-PAGE shows the V(V)-containing sTf species to have comparable stability as apo-sTf likely because of facile V(V) dissociation [67]. There is evidence to suggest that instead of being bound by sTf, V(V) will transform into the anionic vanadate species H2VO4 -and use anionic channels to enter cells ( Figure 15C) [68,102]. H2VO4 -may retain its V(V) oxidation state or be reduced to V(IV) but either form would mimic phosphate and be able to inhibit PTP1B [67].…”

“…This particular coordination modality is expected of ATP for all hard Lewis acidic metal ions. Lay et al have recently found that citrate may not be a sufficient chelator to remove VO 2+ from sTf (bound at the metal binding site or at the surface of Fe2-sTf) within the endosome resulting in resurfacing of the ion following the endocytosis process [68]. The lability of the VO 2+ species released from sTf is important because although the chelated metal center is relatively redox inert, the VO 2+ ion is divalent and can be transported out of the endosome via the DMT1 [112] and be available to inhibit cytosolic PTP1B ( Figure 15C).…”