STABILIZATION OF MICROTUBULE DYNAMICS BY ESTRAMUSTINE BY BINDING TO ANOVEL SITE IN TUBULIN - A POSSIBLE MECHANISTIC BASIS FOR ITS ANTITUMOR ACTION

The cellular targets for estramustine, an antitumor drug used in the t reatment of hormone-refractory prostate cancel; are believed to be the spindle microtubules responsible for chromosome separation at mitosis . Estramustine only weakly inhibits polymerization of purified tubulin into microtubules by binding to tubulin (K-d, approximate to 30 mu M) at a Site distinct from the colchicine or the vinblastine binding sit es, son-ever, by video microscopy, we find that estramustine strongly stabilizes growing and shortening dynamics at plus ends of bovine brai n microtubules devoid of microtubule-associated proteins at concentrat ions substantially below those required to inhibit polymerization of t he microtubules. Estramustine strongly reduced the rate and extent bot h, of shortening and growing, increased the percentage of time the mic rotubules spent in an attenuated state, neither growing nor shortening detectably, and reduced the overall dynamicity of the microtubules, S ignificantly, the combined suppressive effects of vinblastine and estr amustine on the rate and extent of shortening and dynamicity were addi tive, Thus, like the antimitotic mechanisms of action of the antitumor drugs vinblastine and taxol, the antimitotic mechanism of action of e stramustine may be due to kinetic stabilization of spindle microtubule dynamics, The results may explain the mechanistic basis for the benef it derived from combined use of estramustine with vinblastine or taxol , two other drugs that target microtubules, in the treatment of hormon e-refractory prostate cancer.