RNA Interference against MDR1 but not MRP1 Reverses Drug Resistance in Human Small-Cell Lung Cancer Cells

AIM: Human small-cell lung carcinoma (SCLC) cells can express two drug efflux pumps, MDR1 and MRP1, possibly involved in drug resistance. The aim of this study was to investigate the ex vivo significance of these pumps. METHODS: We transfected small interfering RNA (siRNA) directed against the mRNAs of these two genes into drug-sensitive (H69) and drug-resistant (H69VP) SCLC cells and tested down-regulation of gene expression, cytotoxicity, and drug accumulation by calcein efflux. RESULTS: After 24 h, RT-PCR showed down-regulation of the targeted mRNAs. The 3-day cytotoxicities of etoposide, doxorubicin and vincristine were unchanged in H69 cells compared to mock-transfected controls. For the H69VP cells, drug resistance was reversed by anti-MDR1 siRNA but not anti-MRP1 siRNA. These data were supported by experiments measuring reduced accumulation of the fluorescent dye, calcein acetoxymethyl ester. Compared to H69 cells, accumulation after 15 min was reduced by 80% in H69VP cells. This was reversed by concurrent administration of cyclosporin A or pretreatment with siRNA against MDR1, but not by siRNA against MRP1. CONCLUSION: The pump expressed by MDR1 but not MRP1 is important in drug resistance in these cells.