It is generally accepted that the development of GC, like other cancers, involves multiple steps, including the accumulation of genetic and epigenetic changes. However, the precise mechanism underlying gastric carcinogenesis remains unclear. Therefore, it has been a global research hotspot to looking for new therapeutic targets for GC treatment.
Accumulating evidence has indicated that aberrant expression of miRNAs may be a common mechanism involved in the development of various cancers . Investigation of cancer-specific miRNAs and their targets is necessary for further elucidation of their role in the pathogenesis of tumors, and may be important for the design of novel therapeutic targets [6,8,17]. Although miRNAs have been widely studied in different types of cancers, the knowledge of the aberrant expression and potential function of miRNAs in GC is largely lacking. Accumulating evidence shows that miR-107 is one of the oncogenic RNAs, and overexpression of these RNAs has been reported in several types of human malignant solid tumors. Previously, Inoue et al. found that the mean expression level of miR-107 was significantly higher in the GC tissues compared to that of normal tissues. In the comparison of clinicopathological factors, miR-107 expression showed significant association with depth of tumor invasion, lymph node metastasis and tumor stage. In Kaplan-Meier survival curve analysis, OS and DFS of patients with high miR-107 expression were significantly worse than those of patients with low miR-107 expression. In the Cox multivariate analysis, it was shown that miR-107 expression in GC tissues was an independent prognostic factor for OS and DFS. Their results indicate that miR-107 may be useful as an effective biomarker for prediction of a poor prognosis in GC patients . However, the detailed mechanisms of miR-107 were fewly investigated in GC. Previously, Feng et al. found that miR-107 targeted cyclin-dependent kinase 6 (CDK6) expression, induced cell cycle G1 arrest and inhibited invasion in GC cells . Li et al. found that upregulation of miR-107 induced proliferation in GC cells by targeting the transcription factor FOXO1 . In the present study, we validated that the expression of miR-107 was significantly increased in GC cell line compared with normal controls. Consistent with previous findings from other cancers, such as esophageal cancer, pancreatic cancer and colorectal cancer [9–12], in GC, we also found that miR-107 could remarkably promote cell proliferation and suppress apoptosis. In addition, we found that clone formation rate of miR-107 inhibitor transfected group was significantly lower than that of control group, demonstrating that miR-107 inhibitor significantly inhibited GC cell line colony formation. Thus, our data suggested that miR-107 might play an important role in GC development.
As we know, miRNA functions through interacting with target genes thus the key to explore the mechanism of miRNA is to study the interaction between miRNA and its target genes. In this study, CDK8 was predicted to be the target gene of miR-107 by online biological software, then luciferase reporter vectors containing CDK8 gene 3′-UTR region with miR-107 binding site was constructed and specific binding between miR-107 and CDK8 was verified. The expression level of CDK8 mRNA and protein in miR-107 inhibitor transfected GC cell line was significantly decreased compared with control group,, indicating that miR-107 suppressed CDK8 expression posttranscriptionally. CDK8 is a member of CDK family (CDKs), which is a group of serine-threonine protein kinase and consists of 10 members with different homology. In the past decade, It has been showed that CDKs were excessively activated in different tumors . Preclinical studies have proved that CDKs can promote gene transcription, cell differentiation and angiogenesis . In our study, MTT assay showed that down regulation of CDK8 by siRNA could significantly attenuate the oncogenic effect of miR-107, suggesting that miR-107 promoted the proliferation of GC cells partially by targeting CDK8.