L-Leucine stimulates proliferation and casein secretion in porcine mammary epithelial cells through mTOR-p70S6K-4EBP1 signalling pathway

Milk production determines health status and optimal growth of suckling neonates. Recent studies revealed that the secretion of leucine into sow milk from the mammary gland is significantly lower than the amount taken up by the mammary gland from the blood, suggesting the potential and fundamental role in udder metabolism. However, little is known about the underlying mechanism. In this study, porcine mammary epithelial cells (PMECs) were employed to investigate the effects of leucine on protein synthesis and mammalian target of rapamycin (mTOR) signalling. Results showed that leucine supplementation significantly increased the proliferation of PMECs in a dose-dependent manner from 0 to 1 mmol/l. Secretions of four casein subunits were significantly increased by addition of leucine. The phosphorylation of proteins related to mTOR signalling, including mTOR, eukaryotic translation initiation factor 4E-binding protein, and p70 ribosomal protein S6 kinase, were accordingly upregulated by leucine supplementation. Casein secretion was not affected by leucine treatment when mTOR signalling was inhibited by rapamycin. This suggests that leucine stimulates protein synthesis through the mTOR signalling pathway. These findings facilitate the utilization of dietary leucine supplementation to improve milk production in lactating sows.