Abstract
Background:
The development of multidrug resistance (MDR) in cancer patients is often associated with the action of ATP-Binding Cassette (ABC) transporters such as ABCB1 (P-glycoprotein, P-gp) and ABCG2 (breast cancer resistance protein, BCRP) that actively efflux functionally and structurally unrelated anticancer agents out of cancer cells. In recent years, numerous molecularly targeted agents have been shown capable of selectively reversing MDR mediated by either ABCB1 or ABCG2 in human cancer cell lines. However, both ABCB1 and ABCG2 are known to mediate MDR and compensate each other due to a large overlap in the substrate specificity of both transporters. This prompted us to search for agents that could target both ABCB1 and ABCG2 at nontoxic concentrations.
Methods:
The properties of the specific inhibitor of Smad3 (SIS3) were evaluated using in vitro biochemical and cytotoxicity assays and molecular docking experiments to determine its effect on multidrug resistant cells.
Results:
By monitoring the accumulation of fluorescent substrates of ABCB1 and ABCG2 in MDR cancer cells, we discovered that the drug efflux function of both ABCB1 and ABCG2 can be completely inhibited by SIS3. Immunoblotting data revealed that SISI3 had no significant effect on the protein expression level of either ABCB1 or ABCG2 in human cancer cells. More importantly, we demonstrated that SIS3 enhanced drug-induced apoptosis and reversed MDR in both ABCB1- and ABCG2-overexpressing MDR cancer cells in a concentration-dependent manner. The cytotoxicity experiments indicated that ABCB1- and ABCG2-overexpressing cells are not resistant to SIS3, suggesting that SIS3 is likely to be a high-affinity substrate of ABCB1 and ABCG2. Docking analysis further supports the notion that SIS3 directly competes with the binding of another drug substrate by binding tightly to the substrate-binding site of ABCB1 and ABCG2.
Conclusion:
Together, our study indicated that SIS is an effective modulator of both ABCB1 and ABCG2 that may be utilized to combat ABC transporter-mediated drug resistance in cancer patients.
