miRNA-99b-5p suppresses liver metastasis of colorectal cancer by down-regulating mTOR

Liver metastasis is common in patients diagnosed with colorectal cancer (CRC), and is also correlated with poor outcome. In this study we screened the different expression profiles of microRNAs (miRNAs) on the development of liver metastasis in CRC patients. miR-99b-5p was found to be more than 6-fold higher in primary tumors than in matched liver metastases (P = 0.007). Expression of miR-99b-5p in primary tumors of patients with stage III CRC without liver metastases was higher than in CRC patients with liver metastases (P = 0.028). Up-regulated miR-99b-5p was associated with longer overall survival (P = 0.01). Besides, miR-99b-5p silencing in miR-99b-5p-positive CRC cell lines promoted cell migration and up-regulated mTOR, and vice versa. In addition, luciferase assays demonstrated that miR-99b-5p functioned as a tumor suppressor by targeting mTOR. Taken together, our results demonstrate that miR-99b-5p is differently expressed in primary CRC and liver metastasis and functions as a tumor-suppressive microRNA in metastatic CRC. The miR-99b-5p-mTOR axis may serve as a prognostic factor and therapeutic target for anti-metastatic therapy in CRC patients.