GKN1 expression in gastric cancer cells is negatively regulated by miR-544a.

Gastrokine1 (GKN1), important for maintaining the physiological function of the gastric mucosa, is highly expressed in the stomach of healthy individuals but is down-regulated or absent in gastric tumor tissues and derived cell lines. The mechanisms underlying GKN1 gene inactivation are still unknown. We previously showed that GKN1 downregulation in gastric tumors is likely associated with an epigenetic transcriptional complex that negatively regulates GKN1 expression. In addition, TSA-mediated inhibition of HDACs leads to GKN1 restoration at the transcriptional level, but no at the translational level. These findings led to hypothesize the activation of a second regulatory mechanism microRNAs-mediated, thus resulting in translational repression and gene silencing. Bioinformatic analyses performed with 5 different algorithms highlighted that 4 miRNAs contained a seed sequence for the 3'UTR of GKN1 mRNA. Among these, only two miRNAs, hsa-miR-544a and miR-1245b-3p directly target the GKN1-3'UTR as evaluated by luciferase reporter assays. TaqMan miRNA assay performed on gastric cancer cell lines after TSA treatment showed a stronger increase of miR-544a expression than that of miR-1245b-3p. Finally, co-transfection of AGS cells with GKN1-3'UTR and premiR-544a showed compared to controls, a strong reduction of GKN1 expression both at translational and transcriptional levels. The up-regulation of miR-544a could be crucially involved in the GKN1 translational repression, thus suggesting its potential role as a biomarker and therapeutic target in GC patients. These findings indicate that epigenetic mechanisms leading to the inactivation of GKN1 play a key role in the multi-step process of gastric carcinogenesis and would provide an essential starting point for the development of new therapeutic strategies based on epigenetic targets for alternatives gene.