Suppression of microtubule dynamics by epothilone B is associated with mitotic arrest

The epothilones are a group of novel microtubule-targeted, antimitotic compounds that have a paclitaxel-like, assembly enhancing effect on tubulin in vitro as well as in cultured cells. We hypothesize that epothilones induce mitotic arrest by suppressing microtubule dynamics. To test this hypothesis, we used MCF7 cells stably transfected with GFP-alpha-tubulin to analyze microtubule dynamics at three concentrations of epothilone B, one that induced no mitotic arrest (0.2 nM, 20 h), one that induced one-third maximal mitotic arrest (IC(33,) 2 nM, 20 h), and one that induced half-maximal mitotic arrest (IC(50,) 3.5 nM, 20 h). We found that epothilone B suppressed microtubule dynamics in a concentration-dependent manner coincident with mitotic block. At 0.2 nM epothilone B, dynamics were not significantly altered. At 2 nM epothilone B (IC(33)), the mean growth and shortening rates were decreased by 38 and 27%, respectively. Dynamicity was decreased by 47%. At the IC(50), 80% of the cells had nearly complete stabilization of microtubule dynamics, and no anaphase or telophase figures were observed. Comparison of the effects of epothilone B on microtubule dynamics with those of paclitaxel indicated that both drugs alter the same microtubule dynamic parameters to a similar extent. At the IC(50) for mitotic arrest, dynamicity was reduced by 54% by paclitaxel compared with 62% for epothilone B. In 65% of the cells treated with paclitaxel, the microtubules were completely stabilized. Thus, the effects of epothilone B on microtubule dynamics are remarkably similar to those of paclitaxel, suggesting that both drugs induce mitotic block by a similar mechanism.