Radiosensitizing effect of epothilone B on human epithelial cancer cells

Abstract

A combined modality treatment employing radiation and chemotherapy plays a central role in the management of solid tumors. In our study, we examined the cytotoxic and radiosensitive effect of the microtubule stabilizer epothiloneB on two human epithelial tumor cell lines in vitro and its influence on the microtubule assembly. Cancer cells were treated with epothiloneB in proliferation assays and in combination with radiation in colony-forming assays. For the analysis of ionizing radiation-induced DNA damage and the influence of the drug on its repair a γH2AX foci assay was used. To determine the effect of epothiloneB on the microtubule assembly in cells and on purified tubulin, immunofluorescence staining and tubulin polymerization assay, respectively, were conducted. EpothiloneB induced a concentration- and application-dependent antiproliferative effect on the cells, with IC(50)values in the low nanomolar range. Colony forming assays showed a synergistic radiosensitive effect on both cell lines which was dependent on incubation time and applied concentration of epothiloneB. The γH2AX assays demonstrated that ionizing radiation combined with the drug resulted in a concentration-dependent increase in the number of double-strand breaks and suggested a reduction in DNA repair capacity. EpothiloneB produced enhanced microtubule bundling and abnormal spindle formation as revealed by immunofluorescence microscopy and caused microtubule formation from purified tubulin. The results of this study showed that epothiloneB displays cytotoxic antitumor activity at low nanomolar concentrations and also enhances the radiation response in the tumor cells tested; this may be induced by a reduced DNA repair capacity triggered by epothiloneB. It was also demonstrated that epothiloneB in fact targets microtubules in a more effective manner than paclitaxel.