目的  探讨微RNA let-7a-5p通过调控鼠双微体2（MDM2）/肿瘤蛋白53（p53）信号通路对胃癌细胞凋亡和周期阻滞的作用。方法  收集20对胃癌组织及配对的癌旁组织标本，检测let-7a-5p表达；生物信息学数据库分析let-7a-5p在胃癌中的表达及与预后的关系，预测let-7a-5p的靶基因；构建let-7a-5p过表达和低表达细胞模型；CCK-8、划痕实验、流式细胞术分别检测细胞增殖、迁移、凋亡和周期分布；双荧光素酶报告基因实验验证let-7a-5p与MDM2的靶向关系；Western blotting和Real-time PCR检测MDM2、p53及通路中细胞周期蛋白D1 (Cyclin D1)、细胞髓细胞增生原癌基因（c-myc）、Bcl-2和Bax的表达。结果  与癌旁组织相比，let-7a-5p在胃癌组织中低表达（P＜0.05），且低表达患者预后较差；转染let-7a-5p 模拟物（let-7a-5p mimics）可明显过表达人胃癌细胞HGC-27细胞中let-7a-5p的水平（P＜0.01），而转染let-7a-5p 抑制剂（let-7a-5p inhibitor）可降低人胃癌细胞AGS中let-7a-5p的表达（P＜0.05）。Let-7a-5p过表达可抑制HGC-27的增殖和迁移，促进凋亡和周期阻滞；Let-7a-5p低表达则促进AGS细胞增殖和迁移，抑制凋亡和周期阻滞；Targeiscan数据库和双荧光素酶报告基因实验结果表明，let-7a-5p直接靶向MDM2，负调控其表达；Western blotting和Real-time PCR实验结果表明，let-7a-5p过表达可上调p53和Bax的表达，下调MDM2、Bcl-2、Cyclin D1和c-myc的表达。Let-7a-5p低表达可上调MDM2、Bcl-2、Cyclin D1、c-myc的表达，下调p53、Bax的表达。结论  Let-7a-5p通过抑制MDM2/p53信号通路促进胃癌细胞凋亡和周期阻滞。

Objective  To investigate the role and mechanism of microRNA-let-7a-5p (miRAN-let-7a-5p) in regulating apoptosis and cell cycle arrest in gastric cancer cells via the mouse double minute 2 (MDM2) / tumor protein 53 (p53) signaling pathway. Methods  Twenty pairs of gastric cancer tissues and matched adjacent non-tumor tissues were collected to detect let-7a-5p expression. Bioinformatics databases were used to analyze the expression of let-7a-5p in gastric cancer and its correlation with patient prognosis. Potential target genes of let-7a-5p were predicted；Both overexpression and knockdown cell models of let-7a-5p were established. Cell proliferation, migration, apoptosis, and cell cycle distribution were assessed using the CCK-8 assay, wound healing assay, and flow cytometry, respectively. The targeting relationship between let-7a-5p and MDM2 was verified by a dual-luciferase reporter assay. Western blotting and Real-time PCR were performed to measure the expression of MDM2, p53, and downstream factors, including Cyclin D1,cellular myelocytomatosis oncogene product(c-myc）、tumor protein 53(p53）, Bcl-2 and Bax. Results  Real-time PCR results showed that let-7a-5p was significantly downregulated in gastric cancer tissues compared with adjacent normal tissues (P<0.05), and low let-7a-5p expression was associated with poor prognosis. Transfection with let-7a-5p mimics markedly increased let-7a-5p levels in human gastric cancer cells HGC-27(P<0.01), while let-7a-5p inhibitor reduced its expression in human gastric cancer cells AGS(P<0.05). Overexpression of let- 7a-5p inhibited proliferation and migration, promoting apoptosis and cell cycle arrest in HGC-27 cells. In contrast, knockdown of let-7a-5p enhanced proliferation and migration, suppressed apoptosis and cell cycle arrest in AGS cells. Target prediction and dual-luciferase reporter assays confirmed that let-7a-5p directly targeted MDM2 and negatively regulated its expression. Western blotting and Real-time PCR results demonstrated that let-7a-5p overexpression upregulated p53 and Bax, and downregulated MDM2, Bcl-2, CyclinD1, and c-myc. Conversely, let-7a-5p knockdown increased the expression of MDM2, Bcl-2, Cyclin D1, and c-myc, and decreased p53 and Bax levels. Conclusion  Let-7a-5p promotes apoptosis and cell cycle arrest in gastric cancer cells by suppressing the MDM2/p53 signaling pathway.