Background: Multidrug resistance (MDR) is a major obstacle in cancer treatment. In the present study, six regions of the mdr1 gene associated with transcription control or translation initiation were selected as targets. Six antisense oligonucleotides (ASODN; AS1-AS6) complementary to the corresponding sequence of the mdr1 gene were synthesized to investigate whether or not blocking the transcription control sites with ASODN could reverse MDR and which ASODN had the best efficiency for reversing MDR in breast carcinoma cells.
Methods: Forty-eight hours after transfection, mdr1 mRNA and P-glycoprotein (Pgp) were determined by RT-PCR, flow cytometry and Rhodamine 123 (Rh123) retention assay. The chemosensitivity of the treated cells was evaluated by MTT assay.
Results: A significant reduction in expression of mdr1 mRNA and Pgp was found in four groups (AS1, AS3, AS5 and AS6), accompanying a dysfunction of Pgp. The lowest levels of mdr1 index and Pgp expression were observed in the AS6 group. MTT assay showed that a significant reduction of drug resistance was found in the four groups, especially in the AS6 group, which showed an 8.4-fold reduction in drug resistance for adriamycin and a 10.5-fold reduction in drug resistance for vinblastine.
Discussion: These data suggest that the MDR phenotype of breast carcinoma cells could be reversed by ASODN complementary to the transcription control site or translation initiation region. AS6, which is complementary to the translation initiation codon (ATG) of mdr1 cDNA, has the best reversal efficiency.