Structural basis for stabilization of human telomeric G-quadruplex [d-(TTAGGGT)]4 by anticancer drug epirubicin

Abstract

Anthracycline anticancer drugs show multiple strategies of action on gene functioning by regulation of telomerase enzyme by apoptotic factors, e.g. ceramide level, p53 activity, bcl-2 protein levels, besides inhibiting DNA/RNA synthesis and topoisomerase-II action. We report binding of epirubicin with G-quadruplex (G4) DNA, [d-(TTAGGGT)]4, comprising human telomeric DNA sequence TTAGGG, using 1H and 31P NMR spectroscopy. Diffusion ordered spectroscopy, sequence selective changes in chemical shift (~0.33 ppm) and line broadening in DNA signals suggest formation of a well-defined complex. Presence of sequential nuclear Overhauser enhancements at all base quartet steps and absence of large downfield shifts in 31P resonances preclude intercalative mode of interaction. Restrained molecular dynamics simulations using AMBER force field incorporating intermolecular drug to DNA interproton distances, involving ring D protons of epirubicin depict external binding close to T1-T2-A3 and G6pT7 sites. Binding induced thermal stabilization of G4 DNA (~36 °C), obtained from imino protons and differential scanning calorimetry, is likely to come in the way of telomerase association with telomeres. The findings pave the way for drug-designing with modifications at ring D and daunosamine sugar.