Abstract: Objective To observe the effect of metformin on the proliferation and apoptosis of hepatocellular carcinoma (HCC) cells HepG2, and to explore the relationship between its potential mechanism and miR-194-5p/RNA binding motif protein 6 (RBM6) pathway. 
Methods A total of 100 patients with HCC undergoing surgery were randomly divided into control group and experimental group, with 50 cases in each group. The control group was given placebo orally and the experimental group was given metformin orally. Antigomirna, antigomiR-194-5p, pcDNA 3.1, pcDNA 3.1-RBM6, metformin+si-NC, metformin+si-RBM6, antigomiR-194-5p+si-NC and antigomiR-194-5p+si-RBM6 were transfected into HepG2 cells by liposome method. The expressions of miR-194-5p and RBM6 in serum, tissues and cells were detected by fluorescence quantitative polymerase chain reaction (FQ-PCR).  MTT assay and 5-bromo-2-deoxyuracil staining were used to detect cell proliferation, and apoptosis was detected by flow cytometry. Double luciferase reporter gene assay was used to detect the fluorescence activity of cells, and RBM6 protein was detected by Western blot. 
Results The two-year disease-free survival (DFS) of the experimental group was significantly prolonged, and the expression of serum miR-194-5p decreased significantly after treatment (P＜0.05). In vitro cell study showed that the expression of miR-194-5p decreased, the proliferation ability decreased and the apoptosis ability increased significantly in HepG2 cells treated with metformin (P＜0.05). The expression of miR-194-5p in cancer tissues and HepG2 cells was significantly higher than that in adjacent tissues or normal hepatocytes. Inhibition of miR-194-5p weakened the proliferation and enhanced the apoptosis of HepG2 cells (P＜0.05). miR-194-5p negatively regulated the expression of RBM6. There was a significant negative correlation between miR-194-5p and RBM6 in hepatocellular carcinoma tissues (r=0.672, P＜0.05). Overexpression of RBM6 had a similar function with inhibition of miR-194-5p. Knockdown of RBM6 significantly inhibited metformin and the regulation of miR-194-5p on proliferation and apoptosis of HepG2 cells. 
Conclusion Metformin inhibits the proliferation of hepatocellular carcinoma cells, promotes apoptosis and improves DFS of patients. Its potential mechanism may be related to miR-194-5p/RBM6 pathway.

Key words: liver neoplasms, metformin, apoptosis