Inflammation has been recently acknowledged as a key participant in the physiopathology of oncogenesis and tumor progression. The inflammatory cytokine IL-1 beta has been reported to induce the expression of markers associated with malignancy in breast cancerous cells through Epithelial-Mesenchymal Transition (EMT). Aggressive breast cancer tumors classified as Triple Negative do not respond to hormonal treatment because they lack three crucial receptors, one of which is the estrogen receptor alpha (ER alpha). Expression of ER alpha is then considered a good prognostic marker for tamoxifen treatment of this type of cancer, as the binding of this drug to the receptor blocks the transcriptional activity of the latter. Although it has been suggested that inflammatory cytokines in the tumor micro environment could regulate ER alpha expression, the mechanism(s) involved in this process have not yet been established. We show here that, in a cell model of breast cancer cells (6D cells), in which the inflammatory cytokine IL-1 beta induces EMT by activation of the IL-1 beta/1L-1RI/beta-catenin pathway, the up regulation of TWIST1 leads to methylation of the ESR1 gene promoter. This epigenetic modification produced significant decrease of the ER alpha receptor levels and increased resistance to tamoxifen. The direct participation of IL-1 beta in these processes was validated by blockage of the cytokine-induced signaling pathway by wortmannin inactivation of the effectors PI3K/AKT. These results support our previous reports that have suggested direct participation of the inflammatory cytokine IL-1 beta in the transition to malignancy of breast cancer cells. (C) Elsevier Inc. All rights reserved.