Mdr1 transfection causes enhanced apoptosis by paclitaxel: An effect independent of drug efflux function of p-glycoprotein

Purpose. We previously reported that in patient tumors the expression of th e mdr1 p-glycoprotein (Pgp) resulted in a lower paclitaxel-induced inhibiti on of DNA precursor incorporation, but a higher apoptosis (Clin. Cancer Res . 4:2949-2955, 1998). The present study was to evaluate these findings in a n experimental system where the Pgp effect can be studied without confoundi ng factors such as the intra- and inter-tumor heterogeneity associated with patient tumors. Methods. To separate the effect of Pgp on intracellular paclitaxel accumula tion from its effects on drug sensitivity, we compared the drug activity at various extracellular and intracellular drug concentrations using the huma n breast MCF7 tumor cells and its mdr1-transfected variant BC19 cells. Results. Compared to MCF7 cells, BC19 cells showed a 9-fold higher Pgp leve l and > 13-fold higher mdr1 expression. Intracellular paclitaxel accumulati on was 80-130% lower in BC19 cells when the extracellular concentrations we re less than or equal to 100 nM, but the difference was reduced to < 15% di fferences at higher extracellular concentrations of greater than or equal t o1,000 nM. For the G2/M block effect MCF7 cells were 43-fold more sensitive than BC19 cells at equal extracellular concentration, and 3.5-fold more se nsitive at comparable intracellular concentrations. On the contrary, BC19 c ells were more sensitive to the apoptotic effect, BC19 cells showed equal o r higher apoptosis compared to MCF7 cells at extracellular concentrations a bove 100 nM, and a 30-100% higher apoptosis at comparable intracellular con centrations. Conclusions. These results confirm our previous observations in patient tum ors and indicate that enhanced Pgp expression is associated with enhanced s ensitivity to the apoptotic effect of paclitaxel and reduced sensitivity to its G2/M block effect, via yet-unknown mechanisms that art: unrelated to t he effect of Pgp on intracellular drug accumulation.