Differential responses of mitotic spindle pole formation to microtubule-stabilizing agents epothilones A and B at low concentrations

We previously reported that the microtubule-stabilizing agent docetaxel induced formation of fragile acentrosomal spindle poles but that structurally related paclitaxel did not. In the present study, we examined whether the microtubule-stabilizing agents epothilones A and B, which are structurally similar, affect the centrosome/spindle pole architecture. We investigated mitotic processes in epothilone A or B-treated human MDA-MB-435 cells, in which the centrosomes, spindle poles and mitotic microtubules were simultaneously visualized by GFP-Aurora A kinase. Fluorescence microscopy of metaphase cells indicated that several chromosomes were misaligned away from the metaphase plate when treated with IC50 concentrations of epothilone A or B (4.5 or 2 nM, respectively), suggesting that microtubule dynamics was impaired. Interestingly, epothilone B induced formation of acentrosomal spindle poles, but this effect was not observed for epothilone A. Live-cell imaging showed that aster-like structures ectopically arose around the nuclear envelope at the onset of mitosis in epothilone B-treated cells and that one of these asters became an acentrosomal spindle pole. Aster-like structures also arose in the presence of epothilone A, but they were merged into centrosome-derived spindle poles during prometaphase and completely disappeared until metaphase. These results indicate that the centrosome/spindle pole integrity is strongly affected by epothilone B but is not greatly affected by epothilone A. Our findings show that the two epothilones cause different cellular responses at equipotent concentrations and suggest that they have different mechanisms of activity in cells.