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

Abstract

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-7ADR 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-7ADR 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-7ADR 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-7ADR 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-7ADR 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.

Apoptosis as a Therapeutic Target in Cancer and Cancer Stem Cells: Novel Strategies and Futures Perspectives

“…Paclitaxel (Taxol®), cyclophosphamide and cytosine arabinoside are the only commonly used cytotoxic agents shown to elicit apoptosis in breast cancer cells [111,112]. Quantitation of apoptotic cells was done by monitoring the binding of fluorescein isothiocyanate (FITC)-labelled annexin V (a phosphatidylserine-binding protein) to cells in response to our title compounds as described [113]. The apoptosis study shows that 3, 4 and 5, at their IC50 concentrations, provoke early apoptosis in the cells treated for 24 and 48 h. It is worth pointing out that 3 (entry 6) induces greater apoptosis at 48 h (46.73%) than at 24 h (40.08%).…”

“…In response to 40a, the percentage of apoptotic cells increased, from 1.24% in control cells to a maximum of 59.9% apoptotic cells (24 h) at a concentration equal to its IC50 against the MCF-7 cell line. This is a remarkable property because the demonstration of apoptosis in MCF-7 breast cancer cells by known apoptosisinducing agents has proved to be difficult and only few cytotoxic agents act preferentially through an apoptotic mechanism in human breast cancer cells [113,114]. Finally, a fact worth emphasizing is that 7e (the only compound tested) induced neither toxicity nor death in mice after one-month treatment when administered intravenously twice a week, with a 50 mg/kg dose each time.…”

“…b Determined by flow cytometry: see [16]. c Apoptosis was determined using an annexin V-based assay [113].…”

Benzo-Fused Seven- and Six-Membered Derivatives Linked to Pyrimidines or Purines Induce Apoptosis of Human Metastatic Breast Cancer MCF-7 Cells In Vitro

“…(Saniger et al, 2003a). d Apoptosis was determined using an annexin V-based assay (Chadderton et al, 2000). The data indicate the percentage of cells undergoing apoptosis in each sample.…”

“…Paclitaxel (Taxol ® ), cyclophosphamide and cytosine arabinoside are the only commonly used cytotoxic agents shown to elicit apoptosis in breast cancer cells Milas et al, 1995). Quantitation of apoptotic cells was done by monitoring the binding of fluorescein isothiocyanate (FITC)-labelled annexin V (a phosphatidylserine-binding protein) to cells in response to our title compounds as described (Chadderton et al, 2000). The apoptosis study shows that compounds 3, 6, 8 and 9, at their IC 50 concentrations, provoke early apoptosis in the cells treated for 24 and 48 h. It is worth pointing out that compound 6 (entry 7) induces greater apoptosis at 48 h (46.73%) than at 24 h (40.08%) and so does compound 3 [48 h (53.92%) and 24 h (46.63%), entry 4].…”