Background and Aims:Research has shown that microRNA-9(miR-9)is downregulated in various malignant tumors,but its expression and function in gastric cancer remain unclear.In a previous study,we predicted using bioinformatics methods that Bcl-2-associated athanogene 4(BAG4)might be a target gene of miR-9,and we also found that BAG4 is highly expressed in gastric cancer tissues.Therefore,this study investigated the expression and function of miR-9 in gastric cancer and its relationship with BAG4.Methods:The expression levels of miR-9 in gastric cancer tissues and adjacent non-cancerous tissues,as well as in gastric cancer cell lines and normal gastric mucosal cells,were detected using qRT-PCR.After overexpressing and knocking down miR-9 in gastric cancer cells using miR-9 mimics and inhibitors,cell proliferation was assessed using the CCK-8 assay and colony formation assay,and cell invasion was evaluated using a Transwell invasion assay.The targeting relationship between miR-9 and BAG4 was analyzed using a luciferase reporter assay.Then,the expression of BAG4 in gastric cancer cells transfected with miR-9 mimics or si-BAG4 was detected by qRT-PCR and Western blot,and related functional experiments were performed for validation.Results:The expression of miR-9 was significantly lower in gastric cancer tissues(vs.adjacent non-cancerous tissues)and gastric cancer cell lines(vs.normal gastric mucosal cells)(all P＜0.05).The expression of miR-9 was significantly associated with tumor size,depth of tumor invasion,lymph node metastasis,distant metastasis,and TNM stage in gastric cancer patients(all P＜0.05).Patients with high miR-9 expression had a significantly higher overall survival rate than those with low miR-9 expression(P=0.028).Overexpression of miR-9 in gastric cancer cells significantly reduced proliferation and invasion abilities,while miR-9 knockdown significantly enhanced these abilities(both P＜0.05).The luciferase reporter assay indicated that BAG4 was a downstream target gene of miR-9.In gastric cancer cells,both mRNA and protein expression levels of BAG4 were significantly reduced after transfection with miR-9 mimics or si-BAG4,while both mRNA and protein expression levels of BAG4 were significantly increased after transfection with miR-9 inhibitors(all P＜0.05).Proliferation and invasion abilities of gastric cancer cells transfected with si-BAG4 were significantly reduced,and the co-transfection of miR-9 inhibitors reversed the effects of si-BAG4 on cell proliferation and invasion(all P＜0.05).Conclusion:miR-9 is downregulated in gastric cancer,and its expression is closely related to adverse biological characteristics of gastric cancer.The mechanism of action of miR-9 may involve negatively regulating BAG4 expression,thereby affecting the proliferation and invasion of gastric cancer cells.
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