Gastric cancer is the fourth most prevalent forms of human cancers and the second leading cause of cancer-related death in the world, especially in East Asian countries. Its incidence rate is 20 per 100,000 annually . According to its histological subtypes, gastric cancer can be divided into two groups: the intestinal type and the diffuse type. The former is characterized by expansive growth and liver metastasis; whereas the latter is characterized by infiltrative growth and peritoneal dissemination [2–4]. They are both associated with Helicobacter pylori infection that contributes to more than 80% of cases . Nowadays, gastrectomy remains the mainstay treatment for gastric cancer, but the prognosis for advanced stage patients is still very poor. The median survival time for patients with gastric cancer is only 6–9 months . In China, the 5-year overall survival rate of patients with gastric cancer is lower than 40%, although recent advances in chemotherapy and surgical techniques . This is primarily attributed to the following reasons: lack of diagnostic markers for early detection, weak prognostic value of histological indicators, limited efficiency of current treatment for advanced disease and lack of molecular markers utilized for targeted therapy [8–10]. Therefore, it is of great significance to make a better understanding of gastric carcinogenesis and to identify novel molecular markers for the improvement of clinical management of patients with gastric cancer.
MicroRNAs (miRNAs) are a recently discovered category of small (19 ~ 24 nucleotides), non-protein-coding and single-stranded endogenous RNA molecules . miRNAs function as regulators of approximately 60% protein-coding genes’ expression mainly at the post-transcriptional level by binding to the sequences in the 3′ untranslated regions (3′-UTR) of their targeted mRNAs resulting in translational repression or gene silencing . As they are involved in regulation of wide array of biological processes including cell proliferation, differentiation, apoptosis, metastasis, angiogenesis and immune response, miRNAs have been considered to be new approaches of tumor biomarkers for early cancer diagnosis and prognosis. They may play roles in the development and progression of cancers similar to those played by oncogenes or tumor suppressor genes. Recent studies have identified a number of miRNAs with aberrant expression in gastric cancer. For example, the comparison of miRNAs deregulated in gastric cancer revealed a significant increase of several tumor-associated miRNAs such as miR-21, -25 and -106a and miRNAs from the miR-17-92 cluster ; based on the cluster analyses, eight miRNAs (including miR-100, -143 and −145) were upregulated specifically in diffuse-type, while four miRNA (miR-202, -373, -494 and −498) in intestinal-type gastric cancer . In our previous study, we found that the downregulation of miR-206 was significantly correlated with tumor progression and may be a potent prognostic marker of gastric cancer . According to our literature retrieval, miR-22 has been demonstrated to play important roles in different types of cancer, such as hepatocellular carcinoma, breast cancer, colon cancer, lung cancer, and prostate cancer [16–22]. However, its roles in tumorigenesis of gastric cancer are still unknown. Because of its involvement in several tumors in digestive system (hepatocellular carcinoma and colon cancer), we hypothesized that miR-22 might play a role in gastric cancer. Thus, the aim of the present study was to investigate the expression patterns and clinical implications of miR-22 in gastric cancer.