Role of specific apoptotic pathways in the restoration of paclitaxel-induced apoptosis by valspodar in doxorubicin-resistant MCF-7 breast cancer cells

Antony Chadderton, David J. Villeneuve, Stefan Gluck, Angie F. Kirwan-Rhude, Brian R. Gannon, David E. Blais, Amadeo M. Parissenti

Research output: Contribution to journalArticle

39 Scopus citations


Paclitaxel (Taxol®) kills tumor cells by inducing both cellular necrosis and apoptosis. A major impediment to paclitaxel cytotoxicity is the establishment of multidrug resistance whereby exposure to one chemotherapeutic agent results in cross-resistance to a wide variety of other drugs. For example, selection of MCF-7 breast cancer cells for resistance to doxorubicin (MCF-7 (ADR) cells) results in cross-resistance to paclitaxel. This appears to involve the overexpression of the drug transporter P-glycoprotein which can efflux both drugs from tumor cells. However, MCF-7 (ADR) cells possess a deletion mutation in p53 and have considerably reduced levels of the Fas receptor, Fas ligand, caspase-2, caspase-6, and caspase-8, suggesting that paclitaxel resistance may also stem from a bona fide block in paclitaxel-induced apoptosis in these cells. To address this issue, we examined the ability of the P-glycoprotein inhibitor valspodar to restore paclitaxel accumulation, paclitaxel cytotoxicity, and paclitaxel-induced apoptosis. Compared to drug sensitive MCF-7 cells, MCF-7 ADR cells accumulated >6-fold less paclitaxel, were approximately 100-fold more resistant to killing by the drug, and were highly resistant to paclitaxel-induced apoptosis. In contrast, MCF-7 ADR cells pretreated with valspodar were indistinguishable from drug-sensitive cells in their ability to accumulate paclitaxel, in their chemosensitivity to the drug, and in their ability to undergo paclitaxel-induced apoptosis. Valspodar, by itself, did not affect these parameters. This suggests that the enhancement of paclitaxel toxicity in MCF-7(ADR) cells involves a restoration of apoptosis and not solely through enhanced drug-induced necrosis. Morever, it appears that changes in the levels/activity of p53, the Fas receptor, Fas ligand, caspase-2, caspase-6, or caspase-8 activity have little effect on paclitaxel-induced cytotoxicity and apoptosis in human breast cancer cells.

Original languageEnglish (US)
Pages (from-to)231-244
Number of pages14
JournalBreast cancer research and treatment
Issue number3
StatePublished - May 17 2000



  • Apoptosis
  • Breast cancer
  • P-glycoprotein
  • Paclitaxel
  • Resistance
  • Reversal
  • Valspodar

ASJC Scopus subject areas

  • Oncology
  • Cancer Research

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