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To investigate the effects of low expression of microRNA21 (miR21) on the proliferation and apoptosis of pituitary tumor cell line RC4BC, and to analyze its targeting relationship with phosphatase and tensin homolog deleted on chromosome ten (PTEN) . Methods RC4BC cells in the logarithmic growth phase were divided into two groups. Cells in the silence group were transfected with miR21 inhibitor, while cells in the negative control group were transfected with inhibitor negative control NCinhibitor. RTPCR was used to detect the mRNA of miR21 and PTEN. CCK8 assay was used to observe the cell proliferation ability of the two groups (expressed by OD value）, plate cloning assay was used to observe the cell colony formation ability of the two groups (expressed by colony formation number）, flow cytome⁃ try was used to observe the apoptosis rates of the two groups and to observe the cell cycle distribution. Single cell suspension was prepared from RC4BC cells, and miR21 mimics or NCmimics were cotransfected into RC4BC cells with PTENWT or PTENMUT, respectively. After transfection, the cells were labeled as the miR21 mimics+PTENWT group, NCmim⁃ ics+PTENWT group, miR21 mimics+PTENMUT group, and NCmimics+PTENMUT group, respectively. The dual lu⁃ ciferase reporter gene assay was used to verify the targeting relationship between miR21 and PTEN. Results The relative expression levels of miR21 and PTEN mRNA in RC4BC cells of the silence group were 0. 30±0. 08 and 2. 89±0. 14, re⁃ spectively, while those in the negative control group were 1. 01±0. 02 and 0. 99±0. 03, respectively, with statistically sig⁃ nificant differences (all P<0. 05) . The OD values of RC4BC cells of the silence group were lower than those of the nega⁃ tive control group at 24，48, and 72 h (all P<0. 05) . The number of colony formation of RC4BC cells in the silence group was smaller than that in the negative control group (P<0. 05) . The apoptosis rate of RC4BC cells in the silence group was higher than that in the negative control group (P<0. 05) . The proportions of cells in the G0/G1 phase and S phase were 65. 65%±7. 82% and 19. 25%±3. 70%, in the silence group, respectively, versus 45. 62%±5. 03% and 35. 72%±4. 67%, in the negative control group, respectively, with statistically significant differences (all P<0. 05) . The relative luciferase activity of the miR21 mimics+PTENWT group, NCmimics+PTENWT group, miR21 mimics+PTENMUT group, and NCmimics+PTENMUT group was 0. 39±0. 07，1. 02±0. 03，1. 01±0. 04, and 1. 00±0. 03, respectively, and significant difference was found in the relative luciferase activity between the miR21 mimics+PTENWT group and the other groups (all P<0. 05) . Conclusion Silencing miR21 can inhibit the proliferation and promote apoptosis of RC4BC cells in pi⁃ tuitary tumor cell line, and its mechanism may be related to the targeted regulation of PTEN gene. [ABSTRACT FROM AUTHOR]