Sestrin 2 Regulates Milk Protein Synthesis through a LAT1/mTOR Pathway

Sestrin 2, coded by the SESN2 gene, as a leucine sensor, can recognize and bind intracellular free leucine, regulate the mammalian target of rapamycin (mTOR) signaling pathway, and participate in protein synthesis, but its molecular mechanism in the mammary gland is still unclear. The aim of this study is to investigate the molecular mechanism of SESN2 on milk protein synthesis in bovine mammary epithelial cells (MAC-T) and further confirm its lactation-regulating function in the mouse mammary gland. In this study, it was found that, after SESN2 knockdown (KD), the phosphorylation levels of mTOR protein and downstream signaling molecule S6K were increased and the expression of α- and β-casein was increased in MAC-T cells. At the same time, the expression and distribution of LAT1 (the primary transporter of leucine) in the KD cell membrane increased. In mouse mammary tissue, milk production and the expression of α- and β-casein increased after SESN2 knockout (KO). In addition, the ability of leucine sensing was weakened in SESN2 KO mice, and phosphorylation of the mTOR protein and downstream signaling molecule S6K was decreased after the addition of the LAT1 selective inhibitor BCH to the mouse mammary gland explants. In summary, this is the first demonstration of SESN2-LAT1 cross-talk in milk protein synthesis and lactation. This study provides a new idea for further revealing the regulation mechanism of lactation and milk protein synthesis and provides a new target for the genetic section of dairy cattle.