Multiple mechanisms confer different drug-resistant phenotypes in pancreatic carcinoma cells

Purpose: Drug-resistant phenotypes of cancer cells may be caused by complex multimodal mechanisms of resistance. In order to gain further insighte into these mechanisms, a P-glycoprotein-mediated multidrug-resistant phenotype induced by daunorubicin-selection and an alternative drug resistance due to treatment with mitoxantrone were investigated in pancreatic carcinoma-derived cells. Methods: For assessing cross-resistance against various drugs, cell proliferation assays were performed. Drug accumulation was measured by flow cytometry. Messenger RNA expression was analyzed by Northern blot and RT-PCR, whereas protein expression was determined by Western blot. Catalytic activity of DNA-topoisomerases (Topo) 11 was determined by the decatenation assay. Results: In mitoxantrone-selected EPP85-181RNOV cells a decreased accumulation of mitoxantrone and daunorubicin was observed in the absence of P-glycoprotein, multidrug resistance protein or breast cancer resistance protein over-expression. An approximately twofold decrease of DNA topoisomerase 11 catalytic activity could be observed in both drug-resistance-exhibiting cell lines. The reduction of Topo 11 catalytic activity was reflected by decreased expression of Topo IIalpha and IIbeta mRNAs and proteins. Conclusions: The decreased drug accumulation in EPP85-181RNOV cells indicates that alternative transport events are occurring. The decreased catalytic activity and expression of Topo 11 indicate that modulation of Topo 11 catalytic activity contributes to both drug-resistant phenotypes in pancreatic carcinoma cells.