2-Methoxyestradiol arrests cells in mitosis without depolymerizing tubulin

The endogenous estrogen metabolite 2-methoxyestradiol (2-MeOE2) suppresses experimental tumor growth in vivo and inhibits angiogenesis activity in vitro. Moreover, 2-MeOE2 has been observed to block mitosis in cell cultures. As high concentrations of 2-MeOE2 prevent microtubule assembly in vitro, the mitotic arrest has been attributed to inhibition of tubulin polymerization. Here we report that concentrations of 2-MeOE2 that cause complete metaphasal arrest do not inhibit the assembly of mitotic spindles. In contrast to the chromosomal dispersal seen in cells arrested by the tubulin depolymerizing drug colcemid, the chromosomes of cells treated with 2-MeOE2 remained in the metaphasal plate indicating a functional defect of the mitotic spindle. The 2-MeOE2 arrest resembles those induced by compounds affecting microtubule dynamics such as taxol and vinblastine. The 2-MeOE2 block is also similar to that induced by several anti-calmodulin agents. Given that metaphase to anaphase transition is a calmodulin-dependent step and our observation that 2-MeOE2 inhibits calmodulin activity in vitro, we suggest that the 2-MeOE2 metaphasal arrest may occur via inhibition of calmodulin.