Inhibition of microtubule polymerization by synthetic estrogens: formation of a ribbon structure.

Dienestrol, meso-hexestrol, and dl-hexestrol, synthetic nonsteroidal estrogens, were shown to be inhibitors of microtubule assembly in vitro using microtubule proteins isolated from porcine brains. The order of activity of the synthetic estrogens as inhibitors of microtubule assembly is: dienestrol greater than diethylstilbestrol greater than meso-hexestrol greater than dl-hexestrol greater than isodienestrol. The activity of dienestrol as an inhibitor was of the same order as that of (+)-griseofulvin, as determined by turbidity measurement. Electron microscopic observation revealed that twisted ribbon structures are formed from microtubule proteins in the presence of some synthetic estrogens (dienestrol, meso-hexestrol, and dl-hexestrol).