RUNX3-mediated transcriptional inhibition of Akt suppresses tumorigenesis of human gastric cancer cells.

Activation of Akt signaling pathway has been suggested involving in chemoresistance, metastasis and tumorigenesis of gastric cancer. However, the mechanism of Akt regulation in gastric cancer is not fully understood. RUNX3, which was first identified as a transcription factor, suppresses gastric tumorigenesis through regulating expression of target genes. Here, we found that restoration of RUNX3 significantly downregulates the protein and mRNA expression of Akt1 in gastric cancer cell lines, AGS and SNU-1. Knockdown of RUNX3 upregulates protein and mRNA expression of Akt1 in normal gastric epithelial cell line, GES-1. The negative correlation of RUNX3 and Akt expression and downstream β-catenin/cyclin D1 effectors was further confirmed in AGS and GES-1 cell lines, as well as clinical specimens of gastric cancer. We identified two RUNX3-binding sites in Akt1 promoter and the binding of RUNX3 on Akt1 promoter significantly inhibits Akt1 expression. The RUNX3-mediated inhibition of Akt1 caused β-catenin protein degradation and then cyclin D1 downregulation. Restoration of cyclin D1 reverses cell growth inhibition and G1 phase arrest induced by RUNX3 in gastric cancer cells. Our results show that loss of RUNX3 expression can enhance the Akt1-mediated signaling pathway and promote the tumorigenesis process in human gastric cancer.