Summary To investigate the role of ,B-tubulin isotype composition in resistance to paclitaxel, an anti-microtubule agent, human prostate carcinoma (DU-145) cells were intermittently exposed to increasing concentrations of paclitaxel. Cells that were selected and maintained at 10 nm paclitaxel (Pac-1 0) were fivefold resistant to the drug. Pac-1 0 cells accumulated radiolabelled paclitaxel to the same extent as DU-1 45 cells and were negative for MDR-1. Analysis of Pac-10 and DU-145 cells by flow cytometry showed similar cell cycle patterns. Immunofluorescent staining revealed an overall increase of a-and 1-tubulin levels in Pac-10 cells compared with DU-145 cells. Examination of 3-tubulin isotype composition revealed a significant increase in pill isotype in the resistant cells, both by immunofluorescence and by western blot analysis. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of the isotypes confirmed the increase observed for the ,,, by exhibiting ninefold higher ,,, mRNA levels and also showed fivefold increase of the Plv. transcript. In addition, analysis of paclitaxel-resistant cells that were selected at increasing levels of the drug (Pac 2, 4, 6, 8 and 10) exhibited a positive correlation between increasing P,,, levels and increasing resistance to paclitaxel. Increased expression of specific ,B-tubulin isotypes and subsequent incorporation into microtubules may alter cellular microtubule dynamics, providing a defence against the anti-microtubule effects of paclitaxel and other tubulin-binding drugs.Paclitaxel has gained considerable attention in cancer therapy in recent years and is successfully used in treating a variety of tumours, including those of the breast, ovary and lung. In treatment of prostate cancer, paclitaxel is inactive when used as a single agent (Roth et al, 1993). However, in combination with estramustine, another anti-microtubule agent, paclitaxel has significant activity against hormone refractory prostate cancer (Hudes et al, 1995).Despite its preclinical and clinical success, the exact mechanism of action of paclitaxel is not known. At low concentrations, paclitaxel blocks mitosis by kinetic stabilization of spindle microtubules (Jordan et al, 1993). Paclitaxel differs from the other anti-microtubule agents such as vinblastine and colchicine by causing microtubule polymerization instead of depolymerization.The c4p-tubulin heterodimer is the major component of microtubules. Most of the anti-microtubule agents, including paclitaxel, vinblastine, colchicine and estramustine bind to 3-tubulin. Paclitaxel binding sites on 3-tubulin were identified at the N-terminal 31 amino acids and at residues 217-231 of the protein . These two binding sites are part of the colchicine binding site and are highly conserved among species.Both a-and ,-tubulins are encoded by multigene families and exist as several isotypes in cells. 3-Tubulin exists as six isotypes that are evolutionarily conserved across species and differ from each other predominantly at the carboxy terminus. Sever...

Although treatment with R115777 resulted in partial inhibition of FTase activity in mononuclear cells, it did not exhibit single-agent antitumor activity in patients with previously untreated metastatic pancreatic cancer.

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