TLR4 Signaling Promotes Immune Escape Of Human Colon Carcinoma Cells By Increasing Immunosuppressive Cytokine Production And Apoptosis Resistance

Multiple chronic inflammatory diseases were associated with a high cancer risk, including IBD, hepatitis and H. pylori infection predisposing host organs to malignant transformation. The key molecules involved in inflammation-driven carcinogenesis include TLRs, NF-κB and nitric oxide syntheses (NOSs). Chronic inflammation caused by persistent infection with a parasite, bacterium, or virus is a major driving force in tumor development. Infectious organisms trigger inflammation through activation of receptors that recognize PAMPs, such as cell wall components and nucleic acids. TLRs recognize and interact with PAMPs, resulting in the activation of inflammatory cells and initiation of host responses whose major purpose is to eliminate and kill invading organisms. However, inadequate pathogen eradication, prolonged inflammatory signaling, and defects in anti-inflammatory mechanisms can all lead to chronic inflammation and benefit tumor development. The factors induced by inflammatory stimulation, such as transforming growth factor-β(TGF-β) and vascular endothelial cell growth factor (VEGF), which inhibit dendritic cell activation and impair tumor-specific T cell immunity, lead to immune escape and contribute to cancer.In 1996, Taguchi first found Toll-like receptor (TLR) in the drosophila. In 1997, Medzhitov found the first human TLR. Through database searches, human and mouse TLRs consist of a large family with at least 11 members so far. TLRs are widely expressed in immune cells, especially dendritic cell and macrophage. When people focus on the structure, function and signaling of TLRs in APCs, they found that TLRs were expressed in various cancers and might be associated with tumor immune escape Therefore, the primary objective of this study is to investigate the expression of TLR4 on human colon carcinoma cells and the role of TLR4 in the tumor cell immune escape.In this study, we observed the expression of TLR4 on HT-29 and SW480 human colon carcinoma cells, and found that TLR4 was expressed on human colon carcinoma cells in mRNA and protein line. Then we considered that whether TLR4 is functive in HT-29 and SW480 cells. We found that LPS-induced TLR4 activation of HT-29 and SW480 cells resulted in upregulation of IL-8 production and the activation of NF-κB.Multiple mechanisms have been identified that tumors used to escape from immune rejection. One strategy that tumors use to escape from immune surveillance is the secretion of immunosuppressive factors. These factors may be expressed by the malignant cells themselves or by noncancerous cells present at the tumor site, such as TGF-β, VEGF and IL-8. Our study shows that TLR4 ligand-LPS promotes human colon carcinoma cells’secretion of TGF-β, VEGF and IL-8. Another strategy to escape from the adaptive immune response is apoptosis resistance. Tumor cells with elevated levels of NF-κB are resistant to apoptosis induced by chemotherapy, radiotherapy and TNF-a treatment. In our study, LPS can up-regulate NF-κB and phosphorylated MAPK proteins in human colon carcinoma cells, and inhibition of NF-κB can significantly attenuate TRAIL-induced apoptosis. It means that TLR4 is functionally active; NF-κB inhibits apoptosis induced by TRAIL and is necessary for apoptosis resistance induced by LPS in human colon carcinoma cells. It may induce tumor escape from immune surveillance.