Aberrant expression of the MDR1-encoded P-glycoprotein (P-gp) is often associated with clinical multi-drug resistance (MDR) leading to poor prognosis and failure of chemotherapy. However, the precise and cooperative molecular mechanism responsible for MDR1 transcription and expression in acquired MDR remains elusive. We, herein, demonstrate that Wnt/beta-catenin signal pathway is constitutively activated in Doxorubicin-induced MDR cancer cells, in which nuclear beta-catenin specifically interacts with the transcriptional coactivator CBP in a MEK1/2/ERK1/2 signal-dependent manner. Specific knockdown of both beta-catenin and CBP by RNAi-mediated depletion abrogates MDR1 transcription and expression resulting in a complete reversal of P-gp-dependent efflux function and restoration of sensitivity to the Doxorubincin-induced cytotoxicity. Moreover, following pharmacological disruption of CBP and beta-catenin interaction through inhibition of the MEK1/2/ERK1/2 signal by the specific inhibitor PD98059, MDR1 transcription and its encoded P-gp-dependent function are abolished. These findings conclude that the CBP/beta-catenin complex is a core component of the MDR1 transcriptional "enhancesome".