Helicobacter pylori is the strongest known risk factor for gastric adenocarcinoma, yet only a fraction of infected persons ever develop cancer. One H. pylori determinant that augments cancer risk is the cag pathogenicity island, which translocates bacterial proteins into host cells following adherence. Since contact between H. pylori and gastric epithelial cells is critical for induction of injury, the long-term objective of R01 grant CA 77955 is to define mechanisms through which H. pylori induce epithelial responses with carcinogenic potential. A host molecule that may influence carcinogenesis in conjunction with H. pylori is matrix metalloproteinase-7 (MMP-7). MMP-7 attenuates apoptosis and we have demonstrated that MMP-7 is expressed exclusively in human mucosa colonized by H. pylori cag+ strains and that these strains selectively induce MMP-7 through cag-mediated activation of ERK1/2. Our studies now show that up-regulation of MMP-7 by H. pylori also requires p120-catenin (p120). p120 functions as a transcription factor by forming complexes with Kaiso, a transcriptional repressor implicated in tumorigenesis. Over-expression of p120 relieves Kaiso-mediated transcriptional repression of MMP-7, and our data show that H. pylori-induced up-regulation of MMP-7 is mediated by p120 and Kaiso in vitro and in an ex vivo gastric gland culture model system. Rodent models provide valuable insights into gastric carcinogenesis, and our studies now demonstrate that a rodent-adapted derivative (7.13) of a human H. pylori cag+ strain induces gastric cancer in Mongolian gerbils by 4 weeks and in hypergastrinemic (INS-GAS) mice by 24 weeks. Using this strain as a prototype, we show that strain 7.13 selectively activates epidermal growth factor receptor (EGFR) and ERK1/2, a downstream mediator of EGFR, in vitro. Activation of EGFR attenuates apoptosis in response to strain 7.13 and is required for up-regulation of MMP-7. Since EGFR transactivation can directly phosphorylate p120, our hypothesis is that selective activation of p120-dependent pathways by H. pylori cag+ strains contributes to the augmentation in carcinogenic risk conferred by these strains by attenuating epithelial apoptosis.
Our specific aims are: 1. To define the effects of wild-type and isogenic mutant H. pylori strains on p120-dependent cellular responses related to carcinogenesis in vitro. 2. To define the host signaling pathways that regulate H. pylori-induced p120 activation. 3. To define the effects of p120 and EGFR on H. pylori-induced injury in vivo.
These studies will define bacterial and host factors that influence gastric cancer. Such findings may help to identify H. pylori-infected persons at high risk for gastric cancer, who thereby warrant therapy.
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