MicroRNA-21 Promotes the Viability, Proliferation and Milk Fat Synthesis of Ovine Mammary Epithelial Cells by Targeting PDCD4

MicroRNAs (miRNAs) are short endogenous non-coding RNAs and play important roles in regulating mammary development and activities of ovine mammary epithelial cells (OMECs), which affect the milk yield and milk ingredient contents of ewes. We previously found that miR-21 was highly expressed in ovine mammary tissue, while the regulatory mechanisms of miR-21 underlying mammary development and lactation performance are still unclear. Accordingly, in this study, we investigated the functions of miR-21 in the activities of OMECs, and validated the target relationship of miR-21 with a predicted target gene programmed cell death 4 (<i>PDCD4</i>) by a dual-luciferase reporter assay. Finally, we investigated the regulatory effect of <i>PDCD4</i> on the viability, proliferation and milk fat synthesis of OMECs. The overexpression of miR-21 significantly increased the viability of OMECs, the number and proportion of Edu-labeled positive OMECs, as well as the contents of triglyceride in OMECs. In fact, miR-21 inhibitor obtained opposite results with miR-21 mimics. The results obtained from the dual luciferase report and RT-qPCR assays confirmed that the seed sequence of miR-21 can complementarily combine with the 3'-untranslated regions (3'-UTR) of <i>PDCD4</i>, and miR-21 decreased the luciferase activity of <i>PDCD4</i>. Meanwhile, miR-21 also reduced the expression of <i>PDCD4</i>. These results indicate that <i>PDCD4</i> is a target gene of miR-21. It was further found that <i>PDCD4</i> decreased the viability and triglyceride content of OMECs, and the number and proportion of Edu-labeled positive OMECs. These findings suggest that miR-21 promotes the viability, proliferation and milk fat synthesis of OMECs by down-regulating the expression of <i>PDCD4</i>. The results revealed the regulatory mechanism by which miR-21 affected the activities and milk fat synthesis of OMECs in sheep.