Berberine Induces Toxicity in HeLa Cells through Perturbation of Microtubule Polymerization by Binding to Tubulin at a Unique Site

Berberine has been used traditionally for its diverse pharmacological actions. It exhibits remarkable anticancer activities and is currently under clinical trials. In this study, we report that the anticancer activity of berberine could be partly due to its inhibitory actions on tubulin and microtubule assembly. Berberine inhibited the proliferation of HeLa cells with an IC50 of 18 mu M and induced significant depolymerization of interphase and mitotic microtubules. At its IC50, berberine exerted a moderate G2/M arrest and mitotic block as detected by fluorescence-activated cell sorting analysis and fluorescence microscopy, respectively. In a wound closure assay, berberine inhibited the migration of HeLa cells at concentrations lower than its ICso, indicating its excellent potential as an anticancer agent. In vitro studies with tubulin isolated from goat brain indicated that berberine binds to tubulin at a single site with a K-d of 11 mu M. Berberine inhibited the assembly of tubulin into microtubules and also disrupted the preformed microtubules polymerized in the presence of glutamate and paclitaxel. Competition experiments indicated that berberine could partially displace colchicine from its binding site. Results from fluorescence resonance energy transfer, computational docking, and molecular dynamics simulations suggest that berberine forms a stable complex with tubulin and binds at a novel site 24 angstrom from the colchicine site on the beta-tubulin. Data obtained from synchronous fluorescence analysis of the tryptophan residues of tubulin and from the Fourier transform infrared spectroscopy studies revealed that binding of berberine alters the conformation of the tubulin heterodimer, which could be the molecular mechanism behind the depolymerizing effects on tubulin assembly.