The transcription factor RUNX3 is a gastric tumor suppressor. Tumorigenic Runx3^−/− gastric epithelial cells attach weakly to each other, compared with nontumorigenic Runx3^+/+ cells. We aimed to identify RUNX3 target genes that promote cell-cell contact to improve our understanding of RUNX3's role in suppressing gastric carcinogenesis.

We compared gene expression profiles of Runx3^+/+ and Runx3^−/− cells and observed down-regulation of genes associated with cell-cell adhesion in Runx3^−/− cells. Reporter, mobility shift, and chromatin immunoprecipitation assays were used to examine the regulation of these genes by RUNX3. Tumorigenesis assays and immunohistological analyses of human gastric tumors were performed to confirm the role of the candidate genes in gastric tumor development.

Mobility shift and chromatin immunoprecipitation assays revealed that the promoter activity of the gene that encodes the tight junction protein claudin-1 was up-regulated via the binding of RUNX3 to the RUNX consensus sites. The tumorigenicity of gastric epithelial cells from Runx3^−/− mice was significantly reduced by restoration of claudin-1 expression, whereas knockdown of claudin-1 increased the tumorigenicity of human gastric cancer cells. Concomitant expression of RUNX3 and claudin-1 was observed in human normal gastric epithelium and cancers.

The tight junction protein claudin-1 has gastric tumor suppressive activity and is a direct transcriptional target of RUNX3. Claudin-1 is down-regulated during the epithelial-mesenchymal transition; RUNX3 might therefore act as a tumor suppressor to antagonize the epithelial-mesenchymal transition.