TLR4 Dependent Signaling Pathway In Lipopolysaccharide Induced Human Lung Carcinoma Cells Proliferation

Background:Lung cancer is the number one cancer killer in the whole world. There are about 120 thousand newly increasing clinical cases every year. Each year more people will die from lung cancer than from breast, prostate and bowel cancers combined. Lung cancer is also the highest incidence of cancer in our country , about 40 million new cases each year. The World Health Organisation predicts that by 2025 lung cancer will become the first commonest disease in our country and there will be over 100 thousand newly increasing patients of lung carcinoma a year, if not control smoking and air pollution. At present, many scholars conduct extensive research in biological actions of lung cancer, but its pathogenesis is still unclear. In recent years, the inflammation in tumor growth has got a lot of attention. Chronic infection and inflammation in carcinogenesis and tumor progression is thought to be the most important posteriority and environmental factor. Lung cancer is a kind of disease closely related to environmental exposure. The abnormal lung inflammation may have certain relevance with lung cancer occurrence.Lipopolysaccharide is the major constituent of the bacterial wall of gram-negative cells. It plays a crucial role in chronic inflammation. Lipopolysaccharide recognition and transmembrane signal transduction is the key to cause cell inflammatory effect, mainly through toll like receptor 4. TLR4 is important transmembrane receptors and signal transduction receptors in innate immunity, and is a kind of membrane protein widely existing at the cell surface. TLR4 recognition and then combining with LPS, can activate TLR4 signal transduction pathways. Its classical pathway is: TLR4 combines with its specificity ligand LPS depending on myeloid differentiation protein 88 (MyD88) , then promotes mitogen activated protein kinase (MAPK) and IκB kinases (IKKs) phosphorylation respectively, finally activates activating protein-1 (AP-1) and nuclear factor - kappa B( NF -κB), to cause proinflammatory cytokines secretion, such as tumor necrosis factor alpha(TNF-α), interleukin 6 (IL - 6) and IL-1β, etc and then trigger inflammatory reaction. There is growing evidence that inflammatory reaction is closely related with cancer development and progression. The long-term chronic inflammation is important pathogenic factors of stomach, liver, colorectal and prostate cancer. But it whether have the same relationship with lung cancer still unknow, which need us to research.Objective: Our experiment chooses lung adenocarcinoma A549 lines to study the possible mechanism of lipopolysaccharide in promoting lung cancer cell proliferation: whether LPS activateing TLR4-mediated signaling pathway plays an important role in stimulating cell proliferation, thus to explore specific molecular mechanism during chronic inflammation in lung cancer development and progression.Methods: Using TLR4 natural ligand lipopolysaccharide (LPS) to stimulate lung adenocarcinoma cells A549, MTT assay was employed to determine the effect of LPS on A549 cell proliferation. Flow cytometry were applied to examine the growth of A549 cells after stimulation of LPS. Immunocytochemistry staining was used to evaluate the expressions of TLR4, c-Jun, c-Fos and NF-KB protein in A549 cells treated with different concentrations of LPS.Results: LPS stimulated the proliferation of A549 cells, especially after 24-hour treatment. The ratio of cells in G2/M cell cycle treated with LPS (100 ng/mL) was significant higher than those treated with lower or non-LPS treated groups (P<0.05). The expressions of TLR4, c-Jun, c-Fos and NF-κB detected in A549 cells were gradually increased after stimulated with LPS. In addition, the expressions of TLR4, c-Jun, c-Fos and NF-KB were significantly increased in LPS (100 ng/mL) treated group compared with control group (P<0.05).Conclusion: LPS may promote the proliferation of A549 cells via TLR4 signaling pathways then ultimately activate AP-1( heterodimer of c-Jun and c-Fos) and NF-KB leading to persisted proliferation of human lung carcinaoma.