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 nanopartides (diameter 8.6 nm) 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 K-D = 11 nM. This interaction occurs in two steps: in the first, the C-lobe of the TFe2-NP interacts with R1 in 50 mu s: second-order rate constant, k(1) = 6 x 10(10) M-1 s(-1); first-order rate constant, k(-1) = 9 x 10(4) s(-1); dissociation constant, k(1) = 1.5 mu M. In the second step, the protein/protein adduct undergoes a slow (10,000 s) 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 15 min. 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. (C) 2013 Elsevier B.V. All rights reserved.