Results presented in this study indicate that monensin, a Na+/H+ gradient ionophore, is by itself a potent inhibitor of proliferation of both KB parent and KB/multidrug resistant (MDR) cells. By MTT assay, the ID50 of monensin against both cell lines after a 72 h drug exposure was 0.2±0.04 μg/ml. Following a 1 h exposure of KB parent and KB/MDR cells to 0.1-10 μg/ml- monensin, [3H]thymidine, [3H]uridine, [3H]leucine incorporation into DNA, RNA, and proteins, respectively, was not inhibited, thus suggesting that the mechanism of cytotoxicity of monensin may not be mediated by a direct effect on macromolecular synthesis. In the presence of subtoxic concentrations of monensin (0.05-0.1 μg/ml), the ID50 of doxorubicin against KB/MDR cells after a 72 h drug exposure was reduced from >100 μg/ml to 45 and 18 μg/ml, respectively, while the presence of monensin did not significantly alter doxorubicin cytotoxicity against KB parent cells. In 1 h experiments, the presence of monensin (5 μg/ml) increased the intracellular accumulation of doxorubicin in KB/MDR cells by about two- to threefold but not in KB parent cells. Monensin also markedly reduced doxorubicin efflux from KB/MDR cells. Under the same conditions, monensin enhanced the cellular uptake and cytotoxicity of daunorubicin and hydroxyrubicin in KB/MDR cells but not of the lipophilic anthracycline annamycin. By alkaline elution technique, monensin (5 μg/ml) alone did not induce DNA damage in either KB parent or KB/MDR cells after 1 h of incubation and did not enhance doxorubicin-induced DNA damage in KB parent cells, whereas it significantly increased doxorubicin induced DNA double-strand breaks in KB/MDR cells. Our results indicate that reversal of MDR by monensin may be due to facilitation of drug transport and subsequent enhancement of DNA damage in MDR cells.
ASJC Scopus subject areas
- Cancer Research