Effect of cigarette smoke extract on lipopolysaccha-ride-activated mitogen-activated protein kinase signal transduction pathway in cultured cells

Abstract

Background: Lipopolysaccharide (LPS) forms outer membrane of the wall of Gram-negative cells. LPS can directly cause damage to epithelia of respiratory tract and is the major factor responsible for the chronic inflammation of respiratory passage. The mitogen-activated protein kinase (MAPK) signal transduction pathway of the airway epithelia is intimately associated with the action of LPS. The chronic inflammation of respiratory tract and smoking are interrelated and entwined in the development and progression of chronic lung diseases. This study was designed to examine the effects of cigarette smoke extract (CSE) and LPS on MAPK signal transduction pathway in order to further understand the roles CSE and LPS play in chronic lung inflammation.

Methods: Cultured primary human epithelial cells of airway were divided into four groups according to the stimulants used: blank control group, LPS-stimulation group, CSE-stimulation group and CSE plus LPS group. Western blotting was employed for the detection of phosphorylation level of extracellular-signal-regulated-kinase (ERK(1/2)), p38 MAPK and c-Jun N-terminal kinase (JNK). The expression of cytokines of MAPK transduction pathway (granulocyte-macrophage colony stimulating factor (GM-CSF) and mRNA of IL-8) in the primary epithelial cells of respiratory tract was also determined.

Results: Western blotting revealed that the phosphorylation levels of ERK(1/2), p38 MAPK and JNK were low and 2 hours after the LPS stimulation, the phosphorylation of ERK(1/2), p38 MAPK and JNK were all increased. There was a significant difference in the phosphorylation between the LPS-stimulation group and blank control group (P < 0.05); no significant difference was found between CSE-stimulation group and blank control group (P > 0.05); there was a significant difference between CSE + LPS group and blank control group and between CSE + LPS group and LPS group (P < 0.05). The phosphorylation of CSE-LPS group was higher than that of blank control group but lower than that of LPS group. In blank control group, the expression of IL-8 and GM-CSF mRNA was low in the epithelial cells of airway and the release of IL-8 and GM-CSF was also at a low level. One hour after LPS stimulation, the level of IL-8 mRNA increased (P < 0.05) and reached a peak after 2 hours. On the other hand, GM-CSF mRNA level increased 2 hours after the stimulation (P < 0.05) and reached the highest level 4 hours after the stimulation. Two hours after LPS stimulation, IL-8 and GM-CSF protein level began to rise (P < 0.05), and the level was the highest 8 hours after the stimulation (P < 0.01). Stimulation with CSE alone had no effect on the release of IL-8 and GM-CSF and expression of IL-8 mRNA (P > 0.05), but pre-treatment with CSE could delay the LPS-induced release of IL-8 and GM-CSF and the expression of IL-8 mRNA and its peak was lower.

Conclusions: LPS stimulation can significantly increase the phosphorylation of ERK(1/2), p38 MAPK and JNK in the epithelial cells of airway and activate the MAPK transduction pathway, thereby can activate the downstream signal transduction pathway, and can ultimately result in the release of cytokines by the epithelial cells of airway. CSE can partially abolish the LPS-induced activation of MAPK signal transduction pathway and the expression of cytokines of the pathway, which might contribute to the development and progression of the inflammatory reactions in COPD patients.