| Summary: | 碩士 === 國立臺灣大學 === 免疫學研究所 === 104 === Helicobacter pylori (H. pylori) is one of the most common human pathogens which can be found in the upper gastrointestinal tract. There are about 50% of global population infected, and H. pylori infection is associated with chronic gastritis, peptic ulcer and even gastric cancer. It has been reported that the apoptosis of gastric epithelial cells infected by H. pylori is increased and contributes to the pathogenesis of gastric diseases. Previous studies have shown that, during H. pylori infection, gastric-infiltrating T cells are increased and express TRAIL which is capable of inducing apoptosis. Our lab has reported that human gastric epithelial cells sensitized to H. pylori are conferred susceptibility to TRAIL-mediated apoptosis by enhanced assembly of the TRAIL death-inducing signaling complex (DISC) and caspase-8 activation through down-regulation of the short form of cellular FLICE-inhibitory protein (FLIPS). In the preliminary data, we have found that H. pylori infection suppressed Akt phosphorylation correlated to decreased FLIPS expression. However, the role of Akt pathway in regulating H. pylori-induced TRAIL-mediated apoptosis remains unclear.
In this thesis, we investigated how Akt pathway in gastric epithelial cells is affected by H. pylori followed the altered sensitivity to TRAIL. First, the results indicated that the phosphorylation of Akt and expression of FLIPS are reduced in a time-dependent manner in the presence of H. pylori. We also demonstrated that the activation of Akt is positively correlated to the expression level of FLIPS in AGS cell line through either Akt knockdown by siRNA, Akt inactivation by PI3K inhibitor or Akt overexpression by transfection. Furthermore, H. pylori-induced TRAIL-mediated apoptosis was inhibited by Akt overexpression. Following the proposed mechanism involved in FLIPS degradation through Akt-USP8-AIP4-FLIPS pathway in TRAIL-induced apoptosis, our data revealed that, during H. pylori infection, phosphor-Akt downregulation increased ubiquitin-specific protease 8 (USP8) expression, which decreased the ubiquitination of the FLIPS E3 ubiquitin ligase, atrophin interacting protein 4 (AIP4). More active AIP4 caused the enhanced ubiquitination and degradation of FLIPS and apoptosis induction. In addition, we have examined different upstream molecules of Akt and found that PTEN phosphorylation decreased after H. pylori infection.
Moreover, from the aspect of pathogen, we have found that one of the proteins of cag pathogenicity island, CagE, but not CagA, played an important role in altering host cells sensitivity to TRAIL in the preliminary data. In consistent with this finding, the expression of FLIPS and USP8 and the activation of Akt cannot be altered by △CagE or △CagPAI mutant infection.
Taken together, this study gives us an insight into the underlying mechanism of H. pylori-induced TRAIL-mediated apoptosis and provides the possible candidate molecules for the therapeutic target in H. pylori-associated gastric disease.
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