Helicobacter pylori infection stimulates several intracellular signaling pathways and is accompanied by increased gene expression in gastric epithelial cells. High-density cDNA microarray was used to characterize the mRNA expression profile of genes in human gastric cancer cells (MKN45, AGS) cocultured with H. pylori. Coculture with cag pathogenicity island (PAI)-positive H. pylori (wild-type) significantly upregulated mRNA expression in 8 of 2304 genes tested. In 6 (interleukin-8, I kappaB alpha, A20, ERF-1, keratin K7, glutathione peroxidase) of the 8 genes, up-regulation was confirmed by RT-PCR. In coculture with isogenic cagE-negative mutant (Delta cagE), which encodes a type TV secretion system with other genes in the cag PAI, no significant up-regulation was found. We further analyzed the role of A20. Transfection of expression vector encoding A20 resulted in an inhibition of H. pylori-mediated NF-KB activation, indicating that H, pylori-mediated A20 expression could be a negative regulator of NF-KB activation. Taken together, these results indicate the importance of microarray technology as a tool for analyzing the complex interplay between H. pylori and the host.