Abstract : Antiestrogens and selective estrogen receptor modulators (SERMs) such as tamoxifen are effective in controlling the progression of estrogen receptor (ER)- positive breast tumors to more invasive phenotypes. However, over time, many patients acquire resistance to tamoxifen. The mechanisms underlying tamoxifen resistance remain elusive. Proline-, glutamic acid-, and leucine-rich protein-1) (PELP1) is a novel ER coactivator that plays a role in both genomic and nongenomic actions of ER. PELP1 is predominantly localized in the nucleus in hormonally responsive tissues, but recent studies suggest that PELP1 may be exclusively localized in the cytoplasm in cancer cells. We found that MCF-7 cells engineered to specifically express PELP1 in the cytoplasm (PELP1-cyto), exhibited hypersensitivity to estrogen but resistance to tamoxifen. In addition, PELP1-cyto cells exhibited increased association of PELP1 with Src, enhanced MAPK activation, and constitutive stimulation of AKT. Altered localization of PELP1 was sufficient to trigger PELP1's interaction with the p85 subunit of P13 kinase and P13 activation. Consistent with these findings, human tumors expressing PELP1'S in the cytoplasm showed increased AKT activation. Our results suggest that altered localization of coactivators such as PELP1'S promotes resistance to hormonal therapy, presumably via stimulation of nongenomic estrogen responses such as activation of MAPK and AKT.