Difluoromethylornithine antagonizes taxol cytotoxicity in MCF-7 human breast cancer cells.

Taxol is a naturally occurring anticancer agent. We studied the combined effects of taxol with 0.1 mM of the ornithine decarboxylase inhibitor alpha-difluoromethylornithine (DFMO) in the MCF-7 human breast adenocarcinoma cell line. The effects of taxol on MCF-7 cells were evident at 0.05-1 microM and the half-maximum inhibition was calculated to be 0.05 microM. Although the cells in the control group continued to proliferate during an 8-day growth period, cells in the taxol-treated group showed approximately 78% inhibition on day 6 and approximately 92% inhibition on day 8. The combined effects of different concentrations of taxol with 0.1 mM DFMO for 48 h showed that DFMO reversed the cytotoxicity of taxol. The combined effects of 0.5 microM taxol and 0.1 mM DFMO over an 8-day period resulted in the reversal of taxol cytotoxicity by 74% on the sixth day of culture. Pretreatment and posttreatment with 0.1 mM DFMO protected the MCF-7 human breast adenocarcinoma cells from the cytotoxic effect of taxol. Polyamine levels were inhibited in cells treated with DFMO for 24 h. In a separate experiment, we verified that the addition of exogenous putrescine along with taxol and DFMO to cultures for 48 h restored the cytotoxic effects of taxol. Following exposure to 0.5 microM taxol, over 59% of MCF-7 cells were in G2/M phase. DFMO (0.1 mM) showed only a slight increase in the G1 phase of the cell cycle. However, in cells treated with taxol and DFMO, there was no change in the percent of cells in the G2/M phase compared to taxol-treated cells. Therefore, depletion of cellular polyamines may not interfere with cell cycle changes induced by taxol. Treatment of MCF-7 cells with 0.5 microM taxol resulted in the fragmentation of genomic DNA, indicating apoptosis, whereas the combined effects of taxol with DFMO inhibited DNA fragmentation.