Although many studies reported mechanisms involved in the positive regulation of estrogens (E2) target genes, very little is known concerning the repressive effect of E2. In this study, we explored the molecular mechanisms by which E2 regulates CXCR7 expression in breast cancer cells. Our results show that E2-mediated down-regulation of CXCR7 occurs at the transcriptional level as demonstrated using actinomycin D and requires estrogen receptor alpha (ER alpha). In addition, CXCR7 is a primary ER alpha-target gene because the effect of E2 does not require the synthesis of an intermediary protein as revealed by the translational inhibitor cycloheximide treatment. Using an inhibitor of the NF kappa B pathway and chromatin immunoprecipitation assays, we demonstrated that NF kappa B is necessary for the high expression of CXCR7 gene and is recruited to the proximal promoter of the CXCR7 gene. Interestingly, the chromatin immunoprecipitation analyses also showed that E2-treatment significantly prevented the recruitment of NF kappa B to the promoter. Altogether, our results demonstrate that E2, through ER alpha, directly down-regulates CXCR7 expression by interfering with NF kappa B transcription factor at the promoter level. (C) 2013 Elsevier Inc. All rights reserved.