Stimulation of the endosomal TLR pathway enhances autophagy-induced cell death in radiotherapy of breast cancer

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Toll-like receptors (TLRs), which are mainly expressed in antigen presenting cells, perform a critical role in innate immunity by recognizing the specific structural patterns of pathogens and transducing signals to induce an inflammatory reaction. Although it has been reported that various solid cancers express endosomal TLRs, TLR3, 7, 8, and 9, the cellular and molecular function of TLRs in tumorigenesis has not yet been elucidated. In this report, we identified the expression of TLR3 and TLR7 in the human breast cancer cell line MCF-7 and found that TLRs stimulated with their specific ligand induced an anti-tumoral effect in this cell line. Among four synthetic commercial agonists of TLR3 and 7, Poly(I:C) and imiquimod (IMQ) proved to have superior anti-tumoral activity over the other agonists. A decreased growth rate was observed in MCF-7 cells treated with either TLR agonist. The decreased growth rate was due to autophagy and autophagy-induced cell death because treatment with 3-methyladenine, inhibitor of autophagy rescued the growth rate and increased the expression levels of autophagy-related genes. Moreover, survival of MCF-7 cells significantly decreased when the cells were stimulated simultaneously with TLR agonists and radiation exposure. Therefore, this study can be applied to developing a therapeutic adjuvant of TLR agonists in radiotherapy for radio-resistant breast cancer treatment.