Drug resistance caused by overexpression of P-glycoprotein (P-gp), the MDR1 gene product, limits the therapeutic outcome in cancer patients. Although modulation of multidrug resistance (MDR) by pharmacological agents, antibodies, antisense oligonucleotides, siRNA, and inhibitors of signal transduction have been reported, clinical benefits have been disappointing. We and others have sought to understand the regulation of MDR1 gene expression as another approach to this problem. CtBP1 (C-terminal-binding protein 1) is a transcriptional co-regulator that plays an important role in oncogenesis. Recently, it was reported that CtBP1 could serve as both a co-activator and co-repressor of transcription. In this study, we determined the effect of CtBP1 on transcription of the MDR1 gene. Chromatin immunoprecipitation (ChIP) experiments demonstrated that CtBP1 bound to the promoter and coding regions of the MDR1 gene, but not to the non-coding regions. Knockdown of CtBP1 expression by siRNA repressed the expression of MDR1 gene, as evidenced by decreases in MDR1 mRNA (by RT-PCR) and P-gp expression (by Western blot) in the MDR human breast cancer cell line, MCF-7/AdrR, and the ovarian cancer cell line, A2780Dx. Knockdown of CtBP1 also enhanced the sensitivity of MDR cells to chemotherapeutic drugs including doxorubicin and vinblastine, as determined by MTT assay. To further probe the role of CtBP1 in MDR1 transcription, we co-transfected CtBP1 and MDR1 promoter/luciferase constructs into the human colon cancer cells SW620 and ovarian cancer cells A2780, and assayed promoter activity. Co-transfection of CtBP1 and MDR1 constructs resulted in a ∼3.5-fold increase in activity of the MDR1 promoter, confirming the effect of CtBP1 on transcriptional activation of MDR1 gene. Our results suggest that CtBP1 is an activator of MDR1 transcription, and therefore, inhibiting CtBP1 may represent a novel approach to overcoming drug resistance mediated by P-gp.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]