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Pharmacological resistance is a serious threat to the clinical success of hormone therapy for breast cancer. The antiproliferative response to antagonistic drugs such as tamoxifen (Tam) critically depends on the recruitment of NCoR/SMRT corepressors to estrogen receptor alpha (ERα) bound to estrogen target genes. Under certain circumstances, as demonstrated in the case of interleukin-1β (IL-1β) treatment, the protein Tab2 interacts with ERα/NCoR and causes dismissal of NCoR from these genes, leading to loss of the antiproliferative response. In Tam-resistant (TamR) ER-positive breast cancer cells, we observed that Tab2 presents a shift in mobility on sodium dodecyl sulfate--PAGE (SDS-PAGE) similar to that seen in MCF7 wt upon stimulation with IL-1β, suggesting constitutive activation. Accordingly, TamR treatment with Tab2-specific short interfering RNA, restored the antiproliferative response to Tam in these cells. As Tab2 is known to directly interact with the N-terminal domain of ERα, we synthesized a peptide composed of a 14-aa motif of this domain, which effectively competes with ERα/Tab2 interaction in pull-down and co-immunoprecipitation experiments, fused to the carrier TAT peptide to allow internalization. Treatment of TamR cells with this peptide resulted in partial recovery of the antiproliferative response to Tam, suggesting a strategy to revert pharmacological resistance in breast cancer. Silencing of Tab2 in TamR cells by siRNA caused modulation of a gene set related to the control of cell cycle and extensively connected to BRCA1 in a functional network. These genes were able to discern two groups of patients, from a published data set of Tam-treated breast cancer profiles, with significantly different disease-free survival. Altogether, our data implicate Tab2 as a mediator of resistance to endocrine therapy and as a potential new target to reverse pharmacological resistance and potentiate antiestrogen action. [ABSTRACT FROM AUTHOR]