Characterization of nuclear βII-tubulin in tumor cells:: A possible novel target for taxol

As the subunits of microtubules, alpha- and beta-tubulins have been thought to only exist in the cytoplasm where they are incorporated into microtubules. However, the beta(II) isotype of tubulin has recently been observed in the nuclei of rat kidney mesangial cells [Walss et al., 1999: Cell Motil. Cytoskeleton 42:274-284]. In this study, we detected nuclear beta(II)-tubulin in rat C6 glioma cells, human T98G glioma cells, human MCF-7 breast carcinoma cells, human MDA-MB-435 breast carcinoma cells, and human Hela cervix carcinoma cells. In addition, nuclear beta(II)-tubulin in these cells was found to exist as alphabeta(II) dimers instead of assembled microtubules and appeared to be particularly concentrated in the nucleoli. Several anti-tubulin drugs were used to treat C6 cells to determine their influence on nuclear beta(II)-tubulin. Taxol, a tubulin drug with higher specificity for beta(II)-tubulin than for other beta-tubulin isotypes, irreversibly decreased nuclear beta(II) content in a concentration-dependent manner in C6 cells. Meanwhile, cells were found to be apoptotic as was suggested by the presence of multiple micronuclei and DNA fragmentation. On the other hand, no depletion of nuclear beta(II)-tubulin was observed when C6 cells were incubated with colchicine or nocodazole, two anti-tubulin drugs with higher specificity for the alphabeta(IV) isotype, supporting the hypothesis that drugs with higher specificity for beta(II)-tubulin deplete nuclear beta(II)-tubulin.