Long non-coding RNA MLST Promotes Milk Fat Synthesis in Bovine Mammary Epithelial Cells by Sponging miR-7 to Activate the mTOR Signal Pathway

Long non-coding RNAs (lncRNAs) are noncoding transcripts longer than 200 nucleotides with diverse regulatory roles. LncRNAs are being increasingly recognized for their roles in milk fat synthesis. LOC112442979 is a novel uncharacterized lncRNA located on bovine chromosome 20. The expression level of lncRNA LOC112442979 in the mammary gland tissue of dairy cows was significantly higher during middle lactation than in the dry period, suggesting its potential regulatory role in lactation. Subsequently, we renamed lncRNA Mammary Lipid Synthesis-associated Transcript (MLST). In this study, we find that MLST promotes lipid droplet accumulation and triacylglycerol (TAG) synthesis in bovine mammary epithelial cells (BMECs). Bioinformatics analysis predicted that MLST acts as a molecular sponge for miR-7 and that mTOR is a target gene of miR-7. Overexpression of MLST significantly increases the expression of mTOR both at the mRNA and protein levels, whereas knockdown of MLST has the opposite effect. MLST also significantly promotes the expression of key milk fat metabolism-related genes both at mRNA and protein levels, including PPARγ, SREBP1, FASN, CEBPα, CEBPβ, and ACCα, as well as the protein level of phosphorylated mTOR (p-mTOR) in the mTOR signaling pathway. Furthermore, the inhibitory effect of MLST silencing on milk fat synthesis is reversed by the mTOR activator, MHY1485. These results demonstrate that MLST regulates the mTOR signaling pathway by sponging miR-7, thereby promoting milk fat synthesis. Our findings provide valuable insights into the molecular mechanisms underlying milk quality improvement from the perspective of lncRNA-miRNA-mRNA regulatory network.