As an important index to evaluate the quality of milk, milk fat content directly determines the nutrition and flavor of milk. Recently, growing evidence has suggested that long noncoding RNAs (lncRNAs) play important roles in bovine lactation, but little is known about the roles of lncRNAs in milk fat synthesis, particularly the underlying molecular processes. Therefore, the purpose of this study was to explore the regulatory mechanism of lncRNAs in milk fat synthesis. Based on our previous lncRNA-seq data and bioinformatics analysis, we found that Lnc-TRTMFS (transcripts related to milk fat synthesis) was upregulated in the lactation period compared to the dry period. In this study, we found that knockdown of Lnc-TRTMFS significantly inhibited milk fat synthesis, resulting in a smaller amount of lipid droplets and lower cellular triacylglycerol levels, and significantly decreased the expression of genes related to adipogenesis. In contrast, overexpression of Lnc-TRTMFS significantly promoted milk fat synthesis in bovine mammary epithelial cells (BMECs). In addition, Bibiserv2 analysis showed that Lnc-TRTMFS could act as a molecular sponge for miR-132x, and retinoic acid induced protein 14 (RAI14) was a potential target of miR-132x, which was further confirmed by dual-luciferase reporter assays, quantitative reverse transcription PCR, and western blots. We also found that miR-132x significantly inhibited milk fat synthesis. Finally, rescue experiments showed that Lnc-TRTMFS could weaken the inhibitory effect of miR-132x on milk fat synthesis and rescue the expression of RAI14. Taken together, these results revealed that Lnc-TRTMFS regulated milk fat synthesis in BMECs via the miR-132x/RAI14/mTOR pathway. The interference and overexpression of Lnc-TRTMFS can positively regulate the milk fat synthesis and mTOR pathway in BMECs, while the inhibition and mimics of miR-132x can negatively regulate the milk fat synthesis and mTOR pathway. It was found that Lnc-TRTMFS has a targeted binding relationship with miR-132x, and miR-132x has a targeted binding relationship with RAI14. A ceRNA network of Lnc-TRTMFS-miR-132x-RAI14 was further constructed Lay Summary Milk fat is an important index to evaluate the quality of milk. The content of milk fat directly determines the quality and flavor of milk. Studies have shown that milk components can change with the expression of specific genes and noncoding RNA that regulate it in different lactation periods. In this study, after the interference and overexpression of Lnc-TRTMFS on milk fat metabolism in bovine mammany epithelial cells, we found that Lnc-TRTMFS could positively regulate milk fat synthesis in bovine mammary epithelial cells. The ceRNA network of Lnc-TRTMFS-miR-132x-RAI14 was constructed by software prediction and double fluorescein report test, and the salvage effect of Lnc-TRTMFS on milk fat synthesis was confirmed by salvage test. Most importantly, we found that Lnc-TRTMFS and miR-132x can regulate milk fat by regulating the mTOR pathway by regulating RAI14.
