Altered Toll-Like Receptor Expression and Signaling in Colorectal Cancer Cells promotes Invasiveness

Altered Toll-Like Receptor Expression and Signaling in Colorectal Cancer Cells promotes Invasiveness Toll-like receptors (TLR)s are a family of innate immune receptors which induce potent immune responses upon detection of infection and damage. Activation of TLRs play dual roles in tumorigenesis. TLR3 senses double-stranded RNA (dsRNA) which is typically associated with viral infection, but also senses synthetic RNA analogs, which are applied in clinical trials in cancer immunotherapy. RNA released from cancer cells and dying host cells can also be detected by TLR3. TLR3 is best known to mediate potent anti-tumor immunity since it is an important inducer of type I interferons (IFN)s, but our findings indicate that TLR3 expression on tumor cells can trigger inflammatory responses in the tumor microenvironment that support cancer progression. We have assayed a panel of cancerous intestinal epithelial cell lines (IEC)s and have observed that IECs with metastatic potential express TLR3 and respond to dsRNA addition, in contrast to healthy and non-metastatic IECs. This was interesting since TLR3 activation is known to impair proliferation and induce cell death in many cancer cells through production of IFNs. We found, however, that these cells are resistant to dsRNA-induced cell death, fail to produce IFNs in response TLR3 stimulation, and that dsRNA promoted the invasiveness of metastatic IECs in a TLR3-dependent manner. Metastatic IECs were further found to display altered TLR3 expression, with TLR3 expressed at the cell surface, rather than being expressed inside the cells in endosomes where TLR3 normally resides. Surface expression potentially permits TLR3 to sense extracellular activators. The signaling pathways and responses induced by surface TLR3 appear to differ from those initiated from endosomal TLR3 leading to a more inflammatory response characterized by high levels of the chemokine CXCL10, but not IFNβ. Activating TLR3 on metastatic IECs also promoted the invasiveness of these cells. Our results suggest that altered TLR3 expression in tumor cells can have unexpected detrimental outcomes in CRC progression. Although activating TLR3 on immune cells in the tumor microenvironment may be beneficial in cancer immunotherapy, triggering TLR3 on certain tumor cells may have detrimental effects on cancer progression and may even promote metastasis. Determining the outcome of TLR activation in cancer patients is complicated not only because it is difficult to assay their expression in the tumor microenvironment, but also because subtle alterations in the subcellular expression of these proteins may have consequences for whether the immune response they trigger is beneficial or detrimental.