Role of MD-2 in Tool-like receptor 4-dependent signaling in Helicobacter-pylori-associated gastritis

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Helicobacter pylori (H. pylori) lipopolysccharide (LPS) is one of the major virulence factors in the induction of gastritis. LPS-dependent activation of gastric epithelial cells and infiltrating macrophages leads to secretion of several proinflammatory cytokines. However, little is known about the recognition system of H. pylori LPS in gastric mucosal cells. Recent studies have shown that LPS initiates signal transduction through toll-like receptor (TLR)-4, and this pathway is activated by MD-2. The aim of this study is to investigate TRL-4/MD-2 complex-specific recognition of H. pylori LPS in gastric mucosal cells. Gastric antral biopsy samples were taken from patients with and without H. pylori infection. Four gastric cancer cell lines (MKN-7, MKN-28, MKN-45 and AGS) were used for in vitro study. Expression of TLR-4 and MD-2 in gastric biopsy specimens and gastric cancer cell lines was examined by reverse-transcription polymerase chain reaction and western blot analysis. Localization of TLR-4 in histological sections was also evaluated by immunohistochemistry. Luciferase assay was performed for the assessment of H. pylori LPS-induced NF-kB activation in gastric epithelial cells and transient transfectants expressing TLR-4 and/or MD-2. TLR-4 was constitutively expressed in the stomach with and without H. pylori infection and localized in gastric epithelial cells and infiltrating mononuclear cells. However, the expression of MD-2 in H. pylori-positive subjects was higher than that in H. pylori-negative subjects. In vitro, all gastric epithelial cells expressed both TLR-4 and MD-2 and H. pylori LPS stimulated NF-kB activation in these cells. LPS responses in transfectant expressing both TLR-4 and MD-2 was significantly higher than that expressing only TLR-4. TLR-4/MD-2 complex is essential for the recognition of H. pylori LPS and MD-2 may regulate TLR-4-dependent signaling in the development of H. pylori-associated gastritis.