AbstractBackgroundTheplacenta-specific 1(PLAC1)gene encodes a membrane-associated protein which is selectively expressed in the placental syncytiotrophoblast and in murine fetal tissues during embryonic development. In contrast to its transcriptional repression in all other adult normal tissues, PLAC1 is frequently activated and highly expressed in a variety of human cancers, in particular breast cancer, where it associates with estrogen receptor α (ERα) positivity. In a previous study, we showed that ERα-signaling in breast cancer cells transactivatesPLAC1expression in a non-classical pathway. As the members of the p160/nuclear receptor co-activator (NCOA) family, NCOA1, NCOA2 and NCOA3 are known to be overexpressed in breast cancer and essentially involved in estrogen-mediated cancer cell proliferation we asked if these proteins are involved in the ERα-mediated transactivation ofPLAC1in breast cancer cells.MethodsApplying quantitative real-time RT-PCR (qRT-PCR), Western Blot analysis and chromatin immunoprecipitation, we analyzed the involvement of NCOA1, NCOA2, NCOA3 in the ERα-mediated transactivation ofPLAC1in the breast cancer cell lines MCF-7 and SK-BR-3. RNAi-mediated silencing of NCOA3, qRT-PCR, Western blot analysis and ERα activation assays were used to examine the role of NCOA3 in the ERα-mediated regulation of PLAC1 in further detail. Transcript expression ofNCOA3andPLAC1in 48 human breast cancer samples was examined by qRT-PCR and statistical analysis was performed using Student’st-test.ResultsWe detected selective recruitment of NCOA3 but not NCOA1 or NCOA2 to thePLAC1promoter only in ERα-positive MCF-7 cells but not in ERα-negative SK-BR-3 breast cancer cells. In addition, we demonstrate that silencing of NCOA3 results in a remarkable decrease of PLAC1 expression levels in MCF-7 cells which cannot be restored by treatment with estradiol (E2). Moreover, significant higher transcript levels ofPLAC1were found only in ERα-positive human breast cancer samples which also show aNCOA3overexpression.ConclusionsIn this study, we identified NCOA3 as a selective co-activator of ERα-mediated transactivation ofPLAC1in MCF-7 breast cancer cells. Our data introducePLAC1as novel target gene of NCOA3 in breast cancer, supporting the important role of both factors in breast cancer biology.