Purpose: MicroRNA 125a-5p (miR-125a-5p) has been reported to be a tumor suppressor in malignancies of the breast, ovary, lung, and central nervous system. However, the clinical significance of miR-125a-5p in human gastrointestinal cancer has not been explored. We investigated a tumor inhibitory effect of miR-125a-5p in gastric cancer, focusing in particular on the miR-125a-ERBB2 (HER2, HER-2/neu) pathway. Experimental Design: Quantitative RT-PCR was used to evaluate miR-125a-5p expression in 87

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... gastric cancer cases to determine the clinicopathologic significance of miR-125a-5p expression. The regulation of ERBB2 by miR-125a-5p was examined with precursor miR-125a-transfected cells. Furthermore, we investigated whether miR-125a-5p suppresses proliferation of gastric cancer cells in combination with trastuzumab, a monoclonal antibody against ERBB2. Results: Low expression levels of miR-125a-5p were associated with enhanced malignant potential such as tumor size (P ¼ 0.0068), tumor invasion (P ¼ 0.031), liver metastasis (P ¼ 0.029), and poor prognosis (P ¼ 0.0069). Multivariate analysis indicated that low miR-125a-5p expression was an independent prognostic factor for survival. In vitro assays showed that ERBB2 is a direct target of miR-125a-5p, which potently suppressed the proliferation of gastric cancer cells, and, interestingly, the growth inhibitory effect was enhanced in combination with trastuzumab. Conclusions: miR-125a-5p is a meaningful prognostic marker. Furthermore, miR-125a-5p mimic alone or in combination with trastuzumab could be a novel therapeutic approach against gastric cancer. Clin Cancer Res; 17(9); 2725-33. Ó2011 AACR. Abbreviation: RR, Relative risk. a Tumor invasion of mucosa (m), submucosa (sm), muscularis propria (mp), subserosa (ss), penetration of serosa (se), and invasion of adjacent strucures (si). b P < 0.05.