Previous studies demonstrated that over-expression of enolase α (ENO-1) and its translated form e-Myc promoter binding protein (MBP-1) exert a negative regulatory effect on c-Myc transcription.Therefore, we hypothesized that the down-regulation of c-Myc by either ENO-1 or MBP-1, which is induced by long-term tamoxifen (TAM) treatment, may be a mechanism by which breast cancer cells acquire resistance to TAM therapy.In the present study, the mRNA level of ENO-1 in 244 tumor and normal paired tissue samples and 20 laser-capture-microdissected cell clusters were examined by quantitative real-time PCR analysis.Human breast cancer and normal cells with different estrogen receptor (ER) status and the ENO-1-SiRNA knock-downed MCF-7 cells were used to examine whether the down-regulation of ENO-1 could be utilized as a pharmacological approach for improving the efficacy of TAM treatment.Our in vivo results reveal that the high ENO-1 mRNA expression level showed a stronger correlation with ER+ tumors (>90-fold) than with ER-(>20-fold) tissues.Our results further demonstrate that ENO-1 was transcriptionally up-regulated by ERα and nuclear factor kappa B (NFκB) in response to a non-cytotoxic dose of TAM (10-50 nM) in MCF-7 cells.Pretreatment of MCF-7 cells with the NFκB inhibitor pyrrolidine dithiocarbamate (PDTC) significantly attenuated TAM-induced ENO-1 expression.Transfection of ENO-1-SiRNA alone induced significant cell growth inhibition in addition to augmenting TAM-induced eytotoxicity.These results suggest that the use of agents that antagonize ENO-1 expression might represent a novel approach to overcoming TAM resistance in breast cancer.