Discovering the Biological Significance and Therapeutic Potential of miR-29b-3p in Triple-Negative Breast Cancer

The lack of estrogen or progesterone receptors and absence of HER2 amplification/overexpression in triple-negative breast cancer (TNBC) restricts therapeutic options used in clinical management. MicroRNAs (miRNAs) are small, non-coding transcripts which affect important cellular mechanisms by regulating gene expression at the post-transcriptional level. Among this class, attention was focused on miR-29b-3p with a high profile in TNBC and correlated with the overall survival rates, as TCGA data revealed. This study aims to investigate the implication of the miR-29b-3p inhibitor in TNBC cell lines by identifying a potential therapeutic transcript, improving the clinical outcomes of this disease. The experiments were performed on two TNBC cell lines (MDA-MB-231 and BT549) as in vitro models. An established dose of 50 nM was used for all functional assays performed on the miR-29b-3p inhibitor. A decreased level of miR-29b-3p determined a significant reduction in cell proliferation and colony-forming capacity. At the same time, the changes occurring at the molecular and cellular levels were highlighted. We observed that, when inhibiting the expression level of miR-29b-3p, processes such as apoptosis and autophagy were activated. Further, microarray data revealed that the miRNA expression pattern was altered after miR-29b-3p inhibition, pointing out 8 overexpressed and 11 downregulated miRNAs specific for BT549 cells and 33 upregulated and 10 downregulated miRNAs that were specific for MDA-MB-231 cells. As a common signature for both cell lines, three transcripts were observed, two downregulated, miR-29b-3p and miR-29a, and one upregulated, miR-1229-5p. According to DIANA miRPath, the main predicted targets are related to ECM (extracellular matrix) receptor interaction and TP53 signaling. An additional validation step through qRT-PCR was performed, which showed an upregulation of MCL1 and TGFB1. By inhibiting the expression level of miR-29b-3p, it was shown that complex regulatory pathways targeted this transcript in TNBC cells. Keywords: drug resistance; miR-29b-3p inhibitor; miRNA; triple-negative breast cancer.