Poly I:C enhances cycloheximide-induced apoptosis of tumor cells through TLR3 pathway

Background: Toll-like receptor 3 (TLR3) is a critical component of the innate immune response to dsRNA viruses, which was considered to be mainly expressed in immune cells and some endothelial cells. In this study, we investigated the expression and proapoptotic activity of TLR3 in human and murine tumor cell lines. Methods: RT-PCR and FACS analysis were used to detect expression of TLR3 in various human and murine tumor cell lines. All tumor cell lines were cultured with poly I: C, CHX, or both for 12 h, 24 h, 72 h, and then the cell viability was analyzed with CellTiter 96 (R) AQueous One Solution, the apoptosis was measured by FACS with Annexin V and PI staining. Production of Type I IFN in poly I: C/CHX mediated apoptosis were detected through western blotting. TLR3 antibodies and IFN-beta antibodies were used in Blockade and Neutralization Assay. Results: We show that TLR3 are widely expressed on human and murine tumor cell lines, and activation of TLR3 signaling in cancerous cells by poly I: C made Hela cells (human cervical cancer) and MCA38 cells (murine colon cancer) become dose-dependently sensitive to protein synthesis inhibitor cycloheximide (CHX)-induced apoptosis. Blockade of TLR3 recognition with anti-TLR3 antibody greatly attenuated the proapoptotic effects of poly I: C on tumor cells cultured with CHX. IFN-beta production was induced after poly I: C/CHX treatment and neutralization of IFN-beta slightly reduced poly I: C/CHX -induced apoptosis. Conclusion: Our study demonstrated the proapoptotic activity of TLR3 expressed by various tumor cells, which may open a new range of clinical applications for TLR3 agonists as an adjuvant of certain cancer chemotherapy.