Abstract
Toll-like receptor 4 (TLR4) is a pattern recognition receptors, a member of the Toll-like receptor family and it serves a role in innate and acquired immunity. It has previously been reported that TLR4 was overexpressed in a variety of tumor tissues and cells, including colorectal cancer, gastric cancer and ovarian cancer. In the tumor microenvironment, the TLR4 signaling pathway may be activated in order to upregulate forkhead box P3 (Foxp3) expression in regulatory T cells (Tregs), and thus enhance the immunosuppressive function of Tregs. Also, inflammatory cytokine release would be increased, which promotes tumor immune system evasion. Additionally, it has previously been reported that TLR4 activation may induce histone methylation changes at multiple sites. However, the effects of the alterations to histone methylation in the process of TLR4-associated tumor immune system evasion are not currently known. Histone methylation serves a critical role in regulating gene expression. Abnormal histone methylation is closely associated with tumor development and progression. In order to investigate the epigenetic mechanisms underlying Foxp3 regulation by TLR4, the human lung adenocarcinoma cell line A549 was used. In the present study, it was revealed that the expression level of H3K9me1/2 histone lysine demethylase 3A (KDM3A) was significantly increased following TLR 4 activation in the lung adenocarcinoma A549 cell line, whereas silencing of KDM3A expression led to significantly reduced Foxp3 expression under TLR4 regulation. This result suggests that KDM3A participates in TLR4 regulation of Foxp3 transcription. Additional analysis revealed that during nuclear transport of Foxp3, KDM3A may directly bind to the Foxp3 promoter and activate its transcription. This results in increased secretion of Foxp3-downstream inhibitory cytokines, including transforming growth factor-β1 (TGF-β1), interleukin 35 (IL-35) and heme oxygenase 1 (HO-1), which have immunosuppressive effects and ultimately facilitate the immune escape of lung cancer cells. From the results, the present study concluded that TLR4 activation promoted the expression of H3K9me1/2 demethylase KDM3A. KDM3A bound directly to the Foxp3 promoter and promoted Foxp3 transcription, thereby inducing the secretion of Foxp3-associated downstream inhibitory cytokines (TGF-β1, IL-35, and HO-1), ultimately facilitating the immune system evasion of lung adenocarcinoma.