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Overexpression of PCDHB9 promotes peritoneal metastasis and correlates with poor prognosis in patients with gastric cancer

Gastric cancer (GC) is one of the most common human cancers. Genes expressed only in cancer tissue, especially on the cell membrane, may be useful biomarkers for cancer diagnosis and therapeutics. In the present study, we focused on the PCDHB9 gene, which encodes the transmembrane protein protocadherin B9. Immunohistochemical analysis revealed that 62 (36%) of 173 GC cases were positive for protocadherin B9. Protocadherin B9 staining was mainly observed on the GC cell membrane. Expression of protocadherin B9 was frequently found in intestinal‐type GC and correlated with poor prognosis in patients with intestinal‐type GC. Although PCDHB9 knockdown or forced expression of PCDHB9 did not change cell growth or invasion activity in a GC cell line, cell adhesion to fibronectin was significantly reduced by PCDHB9 knockdown and significantly enhanced by overexpression of PCDHB9. Expression levels of ITGA3, ITGA4, ITGA5, and ITGB1 were significantly reduced by knockdown of PCDHB9 and significantly enhanced by overexpression of PCDHB9. Furthermore, both the number and the size of spheres in culture were significantly decreased by PCDHB9 knockdown and significantly increased by overexpression of PCDHB9. In a peritoneal dissemination mouse model, the weight of the total disseminated nodules of MKN‐74 cells was significantly increased by forced expression of PCDHB9. These results indicate that protocadherin B9 plays an important role in the progression rather than the pathogenesis of intestinal‐type GC. Specific inhibitors of protocadherin B9 may constitute promising anti‐cancer drugs with fewer side‐effects. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.