GlyRS is a new mediator of amino acid-induced milk synthesis in bovine mammary epithelial cells

Amino acids are required for the mammalian target of rapamycin (mTOR) signaling pathway and milk synthesis in bovine mammary epithelial cells (BMECs). However, the mechanism through which amino acids activate this pathway is largely unknown. Here we show that glycyl-tRNA synthetase (GlyRS) mediates amino acid-induced activation of the mTOR-S6K1/4EBP1 pathway, and milk protein and fat synthesis in BMECs. Among 19 aminoacyl-tRNA synthetases, only the mRNA expression of GlyRS and Leucyl-tRNA synthetase (LeuRS) were significantly increased by several amino acids including Met and Leu. We then observed that GlyRS knockdown abolished the stimulation of Met on milk protein and fat synthesis in BMECs, whereas GlyRS overexpression led to more significantly increased milk synthesis in cells treated with Met. By western blotting and qualitative real time-polymerase chain reaction analysis (qRT-PCR) analysis, we next revealed that GlyRS is required for amino acid-induced activation of the mTOR-S6K1/4EBP1 pathway. Thus, this study establishes that GlyRS mediates amino acid-induced activation of the mTOR pathway, thereby regulating milk protein and fat synthesis.