Purpose: We have reported that overexpression of mdr1 P-glycoprotein (Pgp) is associated with a higher sensitivity to paclitaxel-induced apoptosis (1,2). The present study examined the substrate specificity of this phenomenon.
Methods: Two Pgp substrates (vincristine and doxorubicin) and three nonsubstrates (cisplatin, camptothecin. and 5-fluorouracil) were studied. Serum deprivation, known to induce apoptosis, was used as a comparison.
Results: The Pgp nonsubstrates and serum deprivation showed similar overall cytotoxicity and apoptosis in human breast MCF7 cells (with negligible Pgp expression) and its mdr1-transfected subline BC19 cells (with nine-fold higher Pgp expression). In contrast, the overall cytotoxicity and apoptosis of the two Pgp substrates was higher in MCF7 cells. Cotreatment with a Pgp inhibitor, verapamil, abolished the difference in intracellular accumulation of doxorubicin as well as the differences in apoptosis between MCF7 and BC19 cells. This finding confirms that the lower apoptosis of doxorubicin in BC19 cells, in the absence of verapamil, was a result of lower intracellular drug accumulation secondary to high Pgp expression in BC19 cells. In contrast, abolishing the difference in intracellular vincristine concentration by verapamil cotreatment resulted in significantly higher apoptosis in BC19 cells. This finding is identical to our previous finding with paclitaxel, where equal intracellular drug concentration resulted in greater apoptosis in the Pgp-rich BC19 cells.
Conclusions: These data, together with the opposite effects of paclitaxel and vincristine on microtubules (i.e., polymerization versus depolymerization), indicate that the enhanced apoptosis in Pgp-rich cells is specific for antimicrotubule agents but is not related to the polymerization of microtubules.