ASCT2 Is Involved in SARS-Mediated β-Casein Synthesis of Bovine Mammary Epithelial Cells with Methionine Supply

The methionine (Met) uptake into mammary cells depends upon the corresponding amino acid (AA) transporters, which play a regulatory role in the mammary protein production beyond transport. Our previous studies have identified that seryl-tRNA synthetase (SARS) could be a novel mediator to regulate essential AA-stimulated casein synthesis in primary bovine mammary epithelial cells (BMECs). However, the regulatory mechanisms of Met in milk protein production in dairy cows remain further clarified. Here, we aimed to investigate the effects of Met on milk protein synthesis in BMECs and explore the underlying mechanism. The effects of Met on the AA transporter, casein synthesis, and the related signaling pathway were evaluated in the BMECs treated with 0.6 mM Met for 6 h combined with or without the inhibition of AA transporter (ASCT2, a neutral AA transporter) activity by the corresponding inhibitor (GPNA). Besides, the effects of SARS on the cells were mainly evaluated in the BMECs treated with 0.6 mM Met for 6 h together with or without SARS knockdown by RNAi interference. The gene expression of AA transporters and pathway-related genes were analyzed by the real-time quantitative polymerase chain reaction method, and the protein expression of related proteins were determined by the western blot assay. Results showed that 0.6 mM Met remarkably enhanced cell growth and β-casein synthesis compared to the supply of other Met concentrations. Among 13 amino acid transporters, 0.6 mM Met highly increased ASCT2 expression. This Met-stimulated ASCT2 expression and the enhanced mammary intracellular Met uptake were both decreased by the addition of 500 μM GPNA, an inhibitor of ASCT2. In the presence of 0.6 mM Met, the inhibition of ASCT2 activity (by GPNA) and SARS expression (by RNAi) both reduced β-casein synthesis. Additionally, 0.6 mM Met increased the gene expression of mTOR, S6K1, 4EBP1, and Akt; in contrast, the inhibition of ASCT2 by GPNA lowered the gene expression of these four genes. Collectively, this work suggests that ASCT2 is involved in the SARS-mediated Met stimulation of β-casein synthesis through enhancing mammary Met uptake and activating the mTOR signaling pathway in BMECs.