Drug resistance caused by overexpression of P-glycoprotein (P-gp), the MDR1 (ABCB1) gene product, limits the therapeutic outcome. Expression of MDR1 can be induced by divergent stimuli, and involves a number of transcriptional factors. We found that the expression of CtBP1 (C-terminal-binding protein 1), a transcriptional co-regulator, was increased (approximately 4-fold) in human multidrug resistant (MDR) cancer cell lines, NCI/ADR-RES and A2780/DX, as compared to their sensitive counterparts. Silencing of CtBP1 expression by RNAi decreased the MDR1 mRNA and P-gp. Knockdown of CtBP1 also enhanced the sensitivity of MDR cells to chemotherapeutic drugs that are transported by P-gp and increased intracellular drug accumulation. In a reporter gene assay, co-transfection of MDR1 promoter constructs with a CtBP1 expression vector resulted in a approximately 2-4-fold induction of MDR1 promoter activity. CtBP1 appeared to contribute to the activation of MDR1 transcription through directly interacting with the MDR1 promoter, as evidenced by its physical binding to the promoter region of the MDR1 gene in chromatin immunoprecipitation and electromobility shift assays. Histone modifications at the MDR1 promoter, such as mono-methylation, di-methylation, and acetylation of histone H3, were not found to be affected by silencing of CtBP1 expression. Our results reveal a novel role for CtBP1 as an activator of MDR1 gene transcription, and suggest that CtBP1 might be one of the key transcription factors involved in the induction of MDR1 gene. Therefore, CtBP1 may represent a potentially new target for inhibiting drug resistance mediated by overexpression of the MDR1 gene.