Time- and Ratio-Dependent Optimization of miR-141-3p Mimic Transfection in MCF-7 Breast Cancer Cells

Abstract Background: Efficient and reproducible delivery of miRNA mimics is essential for functional studies of miRNA-mediated gene regulation in cancer cells. However, transfection outcomes are highly dependent on experimental parameters, particularly post-transfection time and the ratio between miRNA mimics and transfection reagents. Methods: We optimized transfection conditions for miR-141-3p mimics in the human breast cancer cell line MCF-7 by evaluating three post-transfection time points (24, 48, and 72 h) and three miRNA mimic:Lipofectamine RNAiMAX ratios (6 : 3, 6 : 4, and 6 : 5). Transfection efficiency was assessed by stem-loop RT-qPCR quantification of miR-141-3p. Cellular responses were evaluated through cell density measurements and expression analysis of established miR-141 target mRNAs (EGFR, HMGB1, and KLF12). Results: miR-141-3p expression was significantly increased in mimic-transfected cells compared with mock and blank controls at all time points, displaying a time-dependent pattern with maximal expression at 48 h post-transfection. Target mRNA expression exhibited concordant temporal changes, with the greatest degree of downregulation observed at 48 h. Optimization of the miRNA mimic:Lipofectamine ratio revealed a progressive increase in miR-141-3p expression with higher Lipofectamine amounts, with the 6:5 ratio yielding the highest level of miRNA overexpression and the most pronounced downregulation of target mRNA levels consistent with miRNA-mediated regulatory activity. Cell density analysis indicated moderate, ratio-dependent growth inhibition at higher Lipofectamine amounts, without evidence of overt cytotoxicity. Conclusions: Post-transfection time and miRNA mimic:Lipofectamine ratio critically determine transfection efficiency and downstream regulatory effects in MCF-7 cells. The optimized condition identified here provides a reliable basis for subsequent functional studies of miR-141 in breast cancer.