MicroRNA-221 regulates proliferation of bovine mammary gland epithelial cells by targeting the STAT5a and IRS1 genes

Abstract

MicroRNAs (miRNA) play an essential role in mammary gland development and lactation. Previous studies in cattle have shown that miR-221 is highly expressed in peak compared with early lactation. However, the functions of miR-221 in bovine mammary gland epithelial cells and the mechanisms by which this miRNA affects cell proliferation and milk synthesis remain unclear. We hypothesized that miR-221 targets and modulates the expression of specific genes in the Janus kinase-signal transducer and activator of transcription (JAK-STAT) and phosphatidylinositol 3-kinase-proteinkinase B/mammalian target of rapamycin (PI3K-Akt/mTOR) signaling pathways, which have crucial roles in lactation in cattle. Following transfection of miR-221 into cultured bovine mammary gland epithelial cells, inhibition of cell proliferation and reduced viability of these cells were observed using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and flow cytometry analysis. To elucidate the molecular mechanisms of the effects of miR-221 on cell proliferation, we selected potential candidate genes that can be targeted by miR-221 using bioinformatics prediction tools. The dual luciferase assay revealed that STAT5a, STAT3, and IRS1 interact with miR-221 by its direct binding to the 3'-untranslated regions (UTR) of these genes. Subsequent analysis showed that transfection of a miR-221 mimic resulted in significantly decreased expression of STAT5a and IRS1 at both the RNA and protein levels using quantitative real-time PCR and Western blot analyses. Furthermore, expression levels of the downstream genes SOCS3, AKT3, and mTOR that are regulated by STAT5a and IRS1 in the JAK-STAT and PI3K-Akt/mTOR signaling pathways, were also altered after miR-221 transfection. This is the first study to reveal the mechanisms by which miR-221 inhibits mammary gland epithelial cell proliferation by targeting STAT5a and IRS1, key genes in the PI3K-Akt/mTOR and JAK-STAT signaling pathways.