| The overexpression of P-glycoprotein confers cancer cells with multidrug resistance characterized by resistance to many structurally and functionally unrelated anticancer drugs. P-glycoprotein functions as an energy-dependent drug efflux pump that has a broad substrate specificity. Because P-glycoprotein can actively transport drug out of cells, the intracellular concentration of anticancer drug is kept at sublethal level, by which cancer cells circumvent the attack by anticancer drugs. Clinically, multidrug resistance mediated by P-gp is the major cause for the failure of cancer chemotherapy and the death of cancer patients. One of the ways to reverse cancer multidrug resistance is to use chemicals that can directly block the drug-efflux function of P-glycoprotein. Schisandrin B (SchB) is one of the major components present in a Chinese traditional medicinal herb, Schisandra. In this study, we found that this naturally-occurring compound is a potent and a specific agent able to inhibit the drug-efflux function of P-gp, and therefore, is a candidate agent with potential application in clinical cancer MDR.Effects of Sch B on Cytotoxicity and Cellular Drug Accumulation:The in vitro drug efficacies of SchB were assessed by a panel of MDR cell lines ( K562/ADR, K562/VCR, KBV200) and their parental drug sensitive cells. SchB greatly increased the sensitivities of DMR cells but not their drug sensitive parental cells, toward the cytotoxicities of anticancer drugs such as doxorubicin, paclitaxol, etoposide, vincristine, etc. In the aspect of the efficacies of reversing drug resistance using these MDR cells, SchB is comparable with verapamil, a powerful inhibitor of P-gp's drug efflux function. SchB was further proved to be able to increase the drug accumulation andretention by inhibiting the drug efflux, using the P-gp's substrates adriamycin or rhodamine-123 as indicators. The inhibition of drug efflux of MDR cells by SchB is reversible when SchB is removed from the culture.Effects of SchB on Subcellular Drug Distribution:The in vitro drug efficacies of SchB were also assessed by its effects on the drug distribution in subcellular compartments. MDR cells were characterized by its altered subcellular drug distribution, that contributes to the drug resistance. After incubation of cells with daunorubicin (DNR), the fluorescence of DNR was observed to be diffusely distributed in cytoplasm and predominantly in nuclei of drug sensitive cells, whereas in MDR cells, DNR was only observed to be a few faint fluorescent spots or patches closely surrounding the nuclei and cytoplasmic membrane. When Rh-123 was used as a probe, the patterns of Rh-123 distribution in drug sensitive and resistant cells is similar to those of DNR. SchB could markedly restore the diffuse distribution of DNR and Rh-123 in MDR cells. The effect of SchB on the subcellular distribution of drugs in MDR cells is comparable to that of verapamil. These results implicate that SchB may also reverse drug resistance of MDR cells by restoring the subcellular drug distribution.The Effects of SchB on the Sensitization of MDR Cells toward Adriamycin:Adriamycin alone at 1 and 5 g/ml induced only few percentages of cell apoptosis of MDR cell line KBV200. Adriamycin at the same concentrations in the presence of Sch B, however, drastically increased the apoptotic rates of KBV200 cells, although SchB alone did not induce apoptosis. It is noted that neither Sch B nor verapamil could completely restore the response of KBV200 to adriamycin, indicating the presence of other unknown mechanisms underlying drug resistance.Mechanisms Underlying Sch B in Reversing Cancer MDR : The above studies provided several lines of evidence that SchB is able to act on MDR cells with respect to drug sensitivities, intracellular drug accumulation and retention, and subcellular drugdistribution, implicating that SchB most probably could inhibit P-gp. Nevertheless, it is not known that whether SchB directly interact with the membrane P-gp, or downregulate the... |
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