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 the mdrl p-glycoprotein (Pgp) resulted in a lower paclitaxel-induced inhibition of DNA precursor incorporation, but a higher apoptosis (Clin. Cancer Res. 4:2949-2955, 1998). The present study was to evaluate these findings in an experimental system where the Pgp effect can be studied without confounding factors such as the intra- and inter-tumor heterogeneity associated with patient tumors.
METHODS: To separate the effect of Pgp on intracellular paclitaxel accumulation from its effects on drug sensitivity, we compared the drug activity at various extracellular and intracellular drug concentrations using the human breast MCF7 tumor cells and its mdr1-transfected variant BC19 cells.
RESULTS: Compared to MCF7 cells, BC19 cells showed a 9-fold higher Pgp level and >13-fold higher mdrl expression. Intracellular paclitaxel accumulation was 80-130% lower in BC19 cells when the extracellular concentrations were < or = 100 nM, but the difference was reduced to <15% differences at higher extracellular concentrations of > or = 1,000 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 sensitive at comparable intracellular concentrations. On the contrary. BC19 cells were more sensitive to the apoptotic effect: BC19 cells showed equal or higher apoptosis compared to MCF7 cells at extracellular concentrations above 100 nM, and a 30-100% higher apoptosis at comparable intracellular concentrations.
CONCLUSIONS: These results confirm our previous observations in patient tumors and indicate that enhanced Pgp expression is associated with enhanced sensitivity to the apoptotic effect of paclitaxel and reduced sensitivity to its G2/M block effect, via yet-unknown mechanisms that are unrelated to the effect of Pgp on intracellular drug accumulation.