Solution studies on the complex of 4'-epiadriamycin-d-(CGATCG)2 followed by time-resolved fluorescence measurement, diffusion ordered spectroscopy and restrained molecular dynamics simulations

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

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.

Dual mode of binding of anti cancer drug epirubicin to G-quadruplex [d-(TTAGGGT)]4 containing human telomeric DNA sequence induces thermal stabilization

Epirubicin exerts its anti cancer action by blocking DNA/RNA synthesis and inhibition of topoisomerase-II enzyme. Recent reports on its influence on telomere maintenance, suggest interaction with G-quadruplex DNA leading to multiple strategies of action. The binding of epirubicin with parallel stranded inter molecular G-quadruplex DNA [d-(TTAGGGT)]₄ comprising human telomeric DNA sequence TTAGGG was investigated by absorption, fluorescence, circular dichroism and nuclear magnetic resonance spectroscopy. The epirubicin binds as monomer to G-quadruplex DNA with affinity, K_b1 = 3.8 × 10⁶ M^-1 and K_b2 = 2.7 × 10⁶ M^-1, at two independent sites externally. The specific interactions induce thermal stabilization of DNA by 13.2-26.3 °C, which is likely to come in the way of telomere association with telomerase enzyme and contribute to epirubicin-induced apoptosis in cancer cell lines. The findings pave the way for drug designing in view of the possibility of altering substituent groups on anthracyclines to enhance efficacy using alternate mechanism of its interaction with G4 DNA, causing interference in telomere maintenance pathway by inducing telomere dysfunction.