GKN2 Contributes to the Homeostasis of Gastric Mucosa by Inhibiting GKN1 Activity

Gastrokine 1 (GKN1) plays an important role in maintaining gastric mucosa integrity. Here, we investigated whether gastrokine 2 (GKN2) contributes to the homeostasis of gastric epithelial cells by regulating GKN1 activity. We analyzed cell viability, proliferation, and death in AGS cells transfected with GKN1, GKN2, GKN1 plus GKN2 using MTT, BrdU incorporation, and apoptosis assays, respectively. In addition, the expression levels of the cell cycle- and apoptosis-related proteins, miR-185, DNMT1, and EZH2 were determined. We also compared the expression of GKN1, GKN2, and CagA in 50 non-neoplastic gastric mucosae and measured GKN2 expression in 169 gastric cancers by immunohistochemistry. GKN2 inhibited anti-proliferative and pro-apoptotic activities, miR-185 induction, and anti-epigenetic modifications of GKN1. There was a positive correlation between GKN1 and GKN2 expression (P=0.0074), and the expression of GKN1, but not GKN2, was significantly lower in Helicobacter pylori CagA-positive gastric mucosa (P=0.0013). Interestingly, ectopic GKN1 expression in AGS cells increased GKN2 mRNA and protein expression in a time-dependent manner (P=0.01). Loss of GKN2 expression was detected in 126 (74.6%) of 169 gastric cancers by immunohistochemical staining and was closely associated with GKN1 expression and differentiation of gastric cancer cells (P=0.0002 and P=0.0114, respectively). Overall, our data demonstrate that in the presence of GKN2, GKN1 loses its ability to decrease cell proliferation, induce apoptosis, and inhibit epigenetic alterations in gastric cancer cells. Thus, we conclude that GKN2 may contribute to the homeostasis of gastric epithelial cells by inhibiting GKN1 activity. J. Cell. Physiol. 229: 762-771, 2014. (c) 2013 Wiley Periodicals, Inc.