Helicobacter pylori (H. pylori), a spiral-shaped Gram-negative bacterium that colonizes the human gastric mucosa, is estimated to inhabit at least half of the world's human population, causes active chronic gastric inflammation1,2, and has been defined as a class one carcinogen3. So far, various in vivo H. pylori infection models including mice, Mongolian gerbils, piglets, dogs, and monkeys have been developed to define the association between H. pylori infection and gastrointestinal diseases4. Among them the Mongolian gerbil is an appropriate animal model of long-term H. pylori infection in which various gastrointestinal diseases including gastric cancer can be studied5. In vitro, H. pylori -infection models have been established using cultured gastric epithelial cells, such as AGS, and subjected to the investigation of intracellular signaling cascades triggered by H. pylori infection6.
H. pylori can be subclassified into cag pathogenicity island (cag PAI) -positive and cag PAI-negative strains based on the presence or absence of the cag PAI, a 40-kb genome fragment containing 31 genes7. The cag PAI encodes type IV secretion system proteins, which export bacterial macromolecules from bacterial cells into host cells1,8. Only strains containing the cag PAI trigger various changes in signaling cascades in gastric epithelial cells, and disturb cellular functions, resulting in multiple alterations in gene expression profiles. Thus, the cag PAI-positive isolates have been shown to be more virulent strains that produce severe pathologic infection in humans than the cag PAI-negative isolates. Indeed several clinical studies have provided evidence linking cag PAI-positive strains to the increased risk of human gastric cancer, compared with cag PAI-negative strains9-12. In vitro, infection with cag PAI-positive strains in AGS human gastric epithelial cells causes characteristic changes in the cellular phenotype called the “hummingbird” phenotype, characterized by the elongation and spreading of cells13. These changes are induced by bacterial macromolecules via the type IV secretion system encoded by the cag PAI. Since infection with cag PAI-positive H. pylori is usually confirmed by the appearance of the hummingbird phenotypical changes in vitro, infection efficacy can be determined by the prevalence of the “hummingbird” morphological change.
H. pylori is a genomically diverse pathogen and several bacterial virulence factors, including the cag PAI, VacA, BabA2, etc., are considered to play key roles in disease pathogenesis. The cag PAI genes are divided into two major groups, a Western group and a Japanese group, in accordance with the entire cag PAI sequences14. It is important to note that the resultant changes in epithelial cellular functions after H. pylori infection may vary according to the types of the H. pylori strains used. Thus the selection of bacterial strain could be one of the key factors for H. pylori studies. In our recent study, we used a Japanese type of cag PAI-positive H. pylori, TN2GF4, which has been shown to cause severe active chronic gastritis, ulcers, intestinal metaplasia, and gastric adenocarcinoma in infected Mongolian gerbils5, and therefore is supposed to be one of the most virulent strains of cag PAI-positive H. pylori.
In our recent study, we demonstrated that cag PAI-positive H. pylori infection induced proinflammatory transcription factor nuclear factor-κB (NF-κB) activation, leading to the aberrant expression of activation-induced cytidine deaminase (AID)15, a member of the cytidine-deaminase family that acts as a DNA/RNA-editing enzyme. AID is an essential enzyme for somatic hypermutation, class switch recombination, and gene conversion, which physiologically occurs in immunoglobulin genes in B cells16. Recent studies, however, demonstrated that inappropriate expression of AID acts as a genomic mutator that contributes to tumorigenesis17-19, and moreover we provided the first evidence that aberrant AID expression caused by H. pylori infection might be a mechanism of mutational accumulation in the gastric mucosa during H. pylori -associated gastric carcinogenesis15.
Here we describe the experimental procedure used to determine how cag PAI-positive H. pylori infection achieves the induction of endogenous AID expression in cultured gastric epithelial cells. We include detailed protocols for the growth and maintenance of H. pylori culture20,21, epithelial cell cultures, co-culture assays, the polymerase chain reaction (PCR), immunoblot analysis, and immunostaining procedures.