Biochemical mechanism of cross-resistance to paclitaxel in a mitomycin c-resistant human bladder cancer cell line

The present study describes the biochemical mechanism(s) of cross-resistance to paclitaxel in a human bladder cancer cell line (J82/MMC-2), which is >9-fold more resistant to mitomycin C (MMC) than parental cells (J82/WT). The IC50 values for paclitaxel in J82/WT and J82/MMC-2 cell lines were 0.7±0.03 and 2.8±0.7 μM, respectively (P<0.05). Thus, the J82/MMC-2 cell line exhibited 4-fold cross-resistance to paclitaxel compared with J82/WT. Intracellular accumulation of [3H]paclitaxel was comparable in J82/WT and J82/MMC-2 cell lines. There were no qualitative or quantitative differences between the J82/WT and J82/MMC-2 cell lines in terms of their α-tubulin and β-tubulin contents. Paclitaxel-induced apoptosis could not be detected in either cell line over a wide range of drug concentrations. These results indicate that cross-resistance to paclitaxel in the J82/MMC-2 cell line is not linked to reduced drug accumulation, increased drug efflux, alterations in tubulin content or reduced paclitaxel-induced apoptosis. Paclitaxel-induced DNA strand breakage, however, determined by alkaline elution, was markedly lower in the J82/MMC-2 cell line than in J82/WT. These results suggest that paclitaxel cross-resistance in J82/MMC-2 may be attributed to reduced paclitaxel-induced DNA strand breakage. The precise mechanism of reduced paclitaxel-induced DNA strand breakage in J82/MMC-2 cell line relative to J82/WT cells, however, remains to be elucidated.