Estramustine (EM) is an anti-microtubule
drug used in the treatment of
hormone-refractory advanced
prostate cancer. Since microtubules are the targets for EM cytotoxicity, we investigated the effects of EM on the
microtubule-associated protein tau to determine what role it may play in drug resistance. We have compared tau expression in human
prostate cancer cells (DU145) and an EM-resistant derived cell line (E4).
Reverse transcriptase polymerase chain reaction has established that tau is expressed in both cell lines but increased 1.9-fold in E4 compared with DU145 cells. This result was confirmed at the
protein level by Western blotting. Tau is a
phosphoprotein, most of its reported phosphorylation sites being
serine or
threonine residues. We have shown, however, that tau is also phosphorylated at
tyrosine residues in DU145 cells and that the
phosphotyrosine level of tau is significantly increased in E4 cells. Moreover, DU145 cells exposed to short term micromolar
drug concentrations enter a phase of microtubule depolymerization, display an increased level of tau phosphorylation and follow a pattern similar to that observed in EM-resistant E4 cells. EM is therefore able to induce a very rapid change in the posttranslational state of tau. Our results show that the acquisition of EM resistance in E4 cells, which is accompanied by changes at the
tubulin level, is also associated with important changes in tau expression and phosphorylation.