Estrogen receptors are indicators of breast cancer adaptability to endocrine therapies, such as tamoxifen. Deficiency or absence of estrogen receptor α (ER-α) in breast cancer cells results in reduced efficacy of endocrine therapy. Here, we investigated the effect of combined tamoxifen and pentadecanoic acid therapy on ER-α-under-expressing breast cancer cells. Drug resistance gene expression patterns were determined by RNA sequencing analysis and in vitro experiments. For the first time, we demonstrate that the combined treatment of pentadecanoic acid, an odd-chain fatty acid, and tamoxifen synergistically suppresses the growth of human breast carcinoma MCF-7 stem cells (MCF-7/SCs), which were found to be tamoxifen-resistant and showed reduced ER-α expression compared with the parental MCF-7 cells. In addition, the combined treatment synergistically induced apoptosis and accumulation of sub-G1 cells and suppressed epithelial-to-mesenchymal transition (EMT). Exposure to this combination induces re-expression of ER-α at the transcriptional and protein levels, along with suppression of critical survival signal pathways, such as ERK1/2, MAPK, EGFR, and mTOR. Collectively, decreased ER-α expression was restored by pentadecanoic acid treatment, resulting in reversal of tamoxifen resistance. Overall, pentadecanoic acid exhibits the potential to enhance the efficacy of endocrine therapy in the treatment of ER-α-under-expressing breast cancer cells.
Keywords: anticancer therapy; breast cancer stem cells; estrogen receptor; pentadecanoic acid; synergistic effect; tamoxifen resistance.