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Xinyang Nie,1– 3,* Jian Liu,1,2,* Daohan Wang,1,2,* Chuan Li,1,2 Yuxin Teng,1,2 Zhufeng Li,1,2 Yangpu Jia,1,2 Peiyao Wang,1,2 Jingyu Deng,2,3 Weidong Li,1,2 Li Lu1,2 1Department of General Surgery, Tianjin Medical University General Hospital, Tianjin, People’s Republic of China; 2Tianjin Medical University, Tianjin, People’s Republic of China; 3Department of Gastric Surgery, Tianjin Medical University Cancer Institute and Hospital, National Clinical Research Center for Cancer, Key Laboratory of Cancer Prevention and Therapy, Tianjin, Tianjin’s Clinical Research Center for Cancer, Tianjin, 300060, People’s Republic of China*These authors contributed equally to this workCorrespondence: Li Lu; Weidong Li, Department of General surgery, Tianjin Medical University General Hospital, 154, Anshan Road, Heping District, Tianjin, 300052, People’s Republic of China, Email luli_1989@126.com; tjmughgs_lwd@163.comBackground: Cisplatin (DDP) resistance in gastric cancer (GC) is likely to come from gastric cancer stem cells (GCSC). It is a new idea to study the mechanism of the DDP-resistance in GCSC from miRNA.Materials and Methods: CD44+ GCSCs and CD44- control cells were constructed based on the HGC27 gastric cancer cell line. DDP sensitivities in CD44+ and CD44- cells were detected via CCK-8 assay. The differential expression of miR-21-5p in these cell lines was detected by RT‒qPCR. The expression levels of downstream TGF-β 2, SMAD2 and SMAD3 were determined through RT‒PCR and Western blotting. A luciferase assay was used to detect the relationship between miR-21-5p and TGFB2, and the TCGA database, clinical data from our centre, and vivo experiment were used for validation. Finally, we knocked down miR-21-5p to detect changes in cisplatin resistance in GCSCs and to verify changes in the levels of downstream pathways in GCSCs.Results: CD44+ GCSCs induced cisplatin resistance compared with CD44- cells. miR-21-5p was highly expressed in GCSCs, and the TGF-β 2/SMAD pathway was also highly expressed. TGFB2 was proven to be a downstream target gene of miR-21-5p and had a positive relationship with it in phenotype. After knockdown of miR-21-5p, the TGF-β 2/SMAD pathway was also inhibited, and the resistance of GCSCs to cisplatin was specifically decreased.Conclusion: MiR-21-5p promotes cisplatin resistance in gastric cancer stem cells by regulating the TGF-β 2/SMAD signalling pathway.Keywords: miR-21-5p, cisplatin resistance, GCSCs, TGF-β 2 pathway