Abstract: Objective To construct a recombinant expression plasmid encoding human miR-21，and explore the effects of miR-21 expression on the proliferation and apoptosis of gastric cancer cells. Methods Double-stranded DNA encoding human miR-21 was designed and used to construct eukaryotic expression plasmid pPG-miR-EGFP.The plasmid was transfected into the human gastric cancer cell line SGC-7901 using LipofectamineTM2000，and the transfection rate was monitored by fluorescence microscopy. Levels of miR-21 were analyzed by RT-PCR in three sets of cells：untransfected cells（control group），cells transfected with empty vector（empty group）， and cells transfected with expression plasmid（expression group）.In addition，cell growth and proliferation were assayed using MTT，and cell cycle progression and apoptosis were analyzed using flow cytometry based on Hoechst 33258 staining.Expression of PTEN，PCNA， PDCD4，bcl-2，and cyclin D1 were analyzed using Western blotting and immunohistochemistry. Results The recombinant expression plasmid pPG-miR-21-EGFP was successfully constructed and transfected into 75% of cells. Levels of miR-21 were significantly lower in the expression group than in the empty or control groups（t=7.12，5.78，P<0.05）.Expression of cyclin D1，PCNA，and bcl-2 was significantly lower in the expression group than in the other two groups（P<0.05），while expression of PDCD4 and PTEN was significantly higher（P<0.05）.Cells in the expression group showed lower growth and higher apoptosis rates than cells in the control group（P<0.05），and flow cytometry revealed a greater proportion of cells in G₁ phase［（69.71±0.314）% vs （50.1±0.331）%］ and a smaller proportion in S phase［（14.68±0.448）% vs （28.47±0.316）%］（P<0.05 in both cases）. Conclusions Recombinant expression plasmid pPG-miR-21-EGFP can down-regulate miR-21 expression in human gastric cancer SGC-7901 cells and suppress their proliferation to some extent.It can also promote apoptosis，such that many cells stop at the G₁ phase and do not proceed to S phase.These effects on growth and apoptosis may be related to down-regulation of cyclin D1，PCNA，and bcl-2 and/or with up-regulation of PDCD4 and PTEN.

Key words: Gastric neoplasms, miR-21, Proliferation, Apoptosis