Toll-like receptor 3 (TLR3) activation induces microRNA-dependent reexpression of functional RARβ and tumor regression

Toll-like receptor 3 (TLR3) is a key effector of the innate immune system against viruses. Activation of TLR3 exerts an antitumoral effect through a mechanism of action still poorly understood. Here we show that TLR3 activation by polyinosinicpolycytidylic acid induces upregulation of microRNA-29b, -29c -148b, and -152 in tumor-derived cell lines and primary tumors. In turn, these microRNAs induce reexpression of epigenetically silenced genes by targeting DNA methyltransferases. In DU145 and TRAMP-C1 prostate and MDA-MB-231 breast cancer cells, we demonstrated that polyinosinicpolycytidylic acid-mediated activation of TLR3 induces microRNAs targeting DNA methyttransf erases, leading to demethylation and reexpression of the oncosuppressor retinoic acid receptor beta (RARβ). As a result cancer cells become sensitive to retinoic acid and undergo apoptosis both in vitro and in vivo. This study provides evidence of an antitumoral mechanism of action upon TLR3 activation and the biological rationale for a combined TLR3 agonist/retinoic acid treatment of prostate and breast cancer.