Objective To investigate the role of microRNA-21 (miRNA-21) in promoting proliferation of Kazakh's esophageal squamous cell carcinoma (ESCC) cell line Eca109 and Kazakh's ESCC tissues, and its effects on the expression of programmed cell death 4(PDCD4)gene.Methods TheEca109cellsweredividedintomiRNA-21Mimicsgroup(transfersequence:5′-UCAACAUCAGUCUGAUAAGCUA-3′),miRNA-21inhibitorgroup(transfersequence:5′-UAGCUUAUCAGACUGAUGUUGA-3′), negative control group (the transfer random sequence)and normal control group (no transfection).The cells were seeded at 4.5 × 105/well in 6-well cell culture plates. According to RNA interference technology,thecells were transfected with LipofectamineTM 2000. After transfection, the cell proliferation was determined by cell number counting.The expression of miRNA-21 was detected by real-time quantitative polymerase chain reaction (qRT-PCR), and the expression of PDCD4 protein was measured by Western blot.Meanwhile, the expression of miRNA-21 and PDCD4 protein were performed in 18 pairs of Kazakh's ESCC and corresponding adjacent normal esophageal mucosa.ResultsAt 48 hour after transfection,compared with normal control group, the cell number increased 36％ (P=0.002) in miRNA-21 Mimics group, decreased 28％ (P=0.002) in miRNA-21 inhibitor group, and did not change significantly in negative control group (P=0.515). At 48 hour after transfection, the relative expression of miRNA-21 in miRNA-21 Mimics group, miRNA-21 inhibitor group, negative control group and normal controlgroup was 0.37±0.10, 9.17± 1.08, 0.74±0.23 and 1.04±0.34,respectively, and the relative protein expression of PDCD4 was 1.47 ± 0.11, 0.61±0.09, 0.89 ±0.12 and 0.79±0.02 accordingly.Compared with normal control group, the relative expression of miRNA-21 decreased (P=0.031) and the relative protein expression of PDCD4 up regulated (P=0.001) in miRNA-21 inhibitor group, the relative expression of miRNA-21 increased (P=0.001) and the relative protein expression of PDCD4 down regulated (P=0.030) in miRNA-21 Mimics group,and there was no significant difference in negative control group (P=0.272 and 0.541).In 16 pairs of Kazakh's ESCC tissues, the expression of miRNA-21 was significantly higher in tumor tissues (0.11 ±0.09) than that of corresponding adjacent esophageal normal tissues (0.03 ± 0.03, P=0.001), while the relative protein expression of PDCD4 was significantly lower (0.92 ± 0.39) than that of corresponding adjacent normal esophageal tissues (1.57 ± 0.80, P=0.004). And the higher expression of miRNA-21, the lower relative protein expression of PDCD4 (r=-0.538, P=0.046).ConclusionmiRNA-21 may promoted the cell proliferation through inhibiting PDCD4 expression,which involved in the pathogenesis of Kazakh's ESCC.