Identification of Milk Fat Metabolism-Related Pathways of the Bovine Mammary Gland during Mid and Late Lactation and Functional Verification of the ACSL4 Gene

Abstract

The concentration of bovine milk fat changes regularly with lactation stages. In particular, milk fat percentage is higher in late lactation than mid lactation. Furthermore, milk fat composition is highly subject to a few genes. Thus, transcriptome sequencing was performed to explore the expression patterns of differentially-expressed genes (DEGs) in the parenchymal mammary gland of Holstein dairy cows between mid and late lactation. The 725 DEGs were screened (fold change > 2 and <i>p</i>-value < 0.05), and the peroxisome proliferator-activated receptor (PPAR) signaling pathway associated with lipid synthesis had a significant variation between the two periods (<i>p</i>-value < 0.05). The activation of the PPAR signal pathway may a key factor in the increasing of milk fat content in late lactation compared to mid lactation. Acyl-CoA synthetase long-chain family member 4 (<i>ACSL4</i>), a member of the PPAR signaling pathway, was upregulated in late lactation compared to mid lactation (<i>p</i> < 0.05). <i>ACSL4</i> catalyzes the activation of long-chain fatty acids for cellular lipid synthesis. However, it remains uncertain that the molecular mechanism of milk fat synthesis is regulated by <i>ACSL4</i> in dairy cows. Subsequently, the function verification of <i>ACSL4</i> was performed in bovine mammary epithelial cells (BMECs). The upregulated expression of <i>ACSL4</i> was accompanied by the increase of the concentration of intracellular triglycerides, whereas knockdown of <i>ACSL4</i> decreased the concentration of intracellular triglycerides, which demonstrated that <i>ACSL4</i> plays an important role in modulating milk fat synthesis. In conclusion, the results displayed that <i>ACSL4</i> expression regulates triglyceride metabolism in ruminant mammary cells.