Toll-like receptors (TLRs) play key roles in both the innate and adaptive immune systems, particularly in inflammatory responses against pathogen infection. Recent evidence showed that functional TLRs are also expressed on a wide variety of tumor cells; however, their activation exhibits either antitumor or pro-tumor effects. In order to investigate the function of TLR signaling pathways in breast cancers, we analyzed the expression of TLRs and their functional status in breast cancer cells by RT-PCR and NF-κB reporter gene assays in response to synthetic ligands for these receptors. We found that only TLR5 signaling is highly responsive in breast cancer cells. The phosphorylation levels of IκB, ERK, and JNK after flagellin (an agonist of TLR5) stimulation were increased in tumor cells. Activation of TLR5 signaling in tumor cells by flagellin induces the secretion of various cytokines and chemokines including interleukin-6, interleukin-8, and TNF-α. Moreover, flagellin/TLR5 signaling in MCF-7 cells inhibits an anchorage-independent growth, a hallmark of tumorigenic transformation. In addition, the secretion of soluble factors induced by flagellin contributed to the growth-inhibitive activity in an autocrine fashion. The inhibitive activity was further confirmed in mouse xenografts of human breast cancer model. Administration of flagellin inhibits tumor growth and therefore prolongs the survival of tumor-bearing mice. These findings indicate that TLR5 activation by flagellin mediates innate immunity to elicit potent antitumor activity in breast cancers, which may serve as a novel therapeutic target for human breast cancer therapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3819.