Objective:To investigate the effects of miRNA-30a-5p (miR-30a-5p) and metadherin (MTDH) on the proliferation, invasion and migration abilities of human breast cancer cells in vitro.Methods:The expression of MTDH in cancer and paracancerous tissues of 112 breast cancer patients in the database and miR-30a-5p in cancer and paracancerous tissues of 103 breast cancer patients in the database were analyzed using data from The Cancer Genome Atlas (TCGA) database. Pearson correlation analysis was used to analyze the correlation between miR-30a-5p and MTDH in 1 222 breast cancer patients in the database; the data were updated to August 2022. Breast cancer MDA-MB-231 cells were divided into negative control group (transfected with negative control sequence), miR-30a-5p overexpression group (transfected with miR-30a-5p mimics), siMTDH group [transfected with small interfering RNA against MTDH (siMTDH)], siMTDH+miR-30a-5p overexpression group (transfected with both siMTDH and miR-30a-5p mimics); cell proliferation ability was detected by methyl thiazol tetrazolium (MTT) assay, cell migration ability was detected by cell scratch assay, cell invasion ability was detected by Transwell assay. The relative expressions of miR-30a-5p, MTDH, matrix metalloproteinase (MMP)-2, MMP-9, vimentin and β-catenin mRNA in cells were detected by quantitative real-time fluorescence polymerase chain reaction (qRT-PCR), and the expressions of MTDH, N-cadherin (N-cad), β-catenin, Snail and MMP-9 proteins were detected by Western blotting.Results:In the TCGA database, MTDH expression level was higher and miR-30a-5p expression level was lower in breast cancer tissues compared with paracancerous tissues, and the differences were statistically significant (both P < 0.001). There was a negative correlation between MTDH and miR-30a-5p expressions in 1 222 patients with breast cancer ( r=-0.134, P < 0.001). Compared with the negative control group, the cell proliferation ability was reduced in both siMTDH group and miR-30a-5p overexpression group at 24, 48 and 72 h (all P < 0.001). The cell scratch healing rate in miR-30a-5p overexpression group and siMTDH group was lower than that in negative control group [(61.6±1.6)%, (54.7±5.9)% vs. (80.3±3.0)%] (both P < 0.05). Compared with the negative control group, The number of migrated cells in miR-30a-5p overexpression group and siMTDH group was less than that in negative control group (881±50, 725±63 vs. 1 172±66) (both P < 0.05). Compared with the negative control group, the relative expressions of MMP-2, MMP-9, vimentin and β-catenin mRNA were all down-regulated in MDA-MB-231 cells of miR-30a-5p overexpression group and siMTDH group (all P < 0.05). Compared with the negative control group, the relative expressions of N-cad, β-catenin, Snail and MMP-9 proteins were down-regulated in MDA-MB-231 cells of miR-30a-5p overexpression group and siMTDH group (all P < 0.05). There was no statistical difference in the number of migrated MDA-MB-231 cells between siMTDH+miR-30a-5p overexpression group and siMTDH group (476±5 vs. 389±46, t = 3.37, P = 0.078). There was no statistical difference in the number of migrated cells between siMTDH+miR-30a-5p overexpression group and miR-30a-5p overexpression group (476±5 vs. 477±22, t = 0.02, P = 0.983). Conclusions:The expression of miR-30a-5p is negatively correlated with the expression of MTDH in breast cancer tissues, and either overexpression of miR-30a-5p or silence of MTDH in breast cancer MDA-MB-231 cells in vitro can inhibit cell proliferation, invasion and migration, but MTDH may not be a target gene of miR-30a-5p.