Effects of glucose availability on αS1-casein synthesis in bovine mammary epithelial cells

Glucose has been demonstrated to affect milk protein synthesis in dairy cows. However, its potential mechanisms has not been thoroughly studied. The objective of this study was to investigate the effects of glucose availability on αS1-casein synthesis, glucose uptake, metabolism, and the expression of proteins involved in AMP-activated protein kinase (AMPK)/mammalian target of rapamycin (mTOR) signaling pathway in bovine mammary epithelial cells (BMEC). BMEC were treated for 24 h with different concentrations of glucose (0, 7, 10.5, 14, 17.5, and 21 mM). The results showed that 10.5 and 14 mM glucose supply increased the expression of αS1-casein, glucose uptake, cellular ATP content, and the phosphorylation of mTOR and P70S6K, but repressed AMPK phosphorylation in BMEC. Compared with 10.5 and 14 mM glucose supply, 17.5 and 21 mM glucose decreased the expression of αS1-casein, P70S6K phosphorylation as well as the activity of hexokinase (HK) and pyruvate kinase (PK), but increased the activity of glucose-6-phosphate dehydrogenase (G6PD). These results indicate that 10.5 to 14 mM glucose supply is the proper range for αS1-casein synthesis, and the promotion effects may be related to the increase of glucose uptake, ATP content and the changes of key proteins' phosphorylation in AMPK/mTOR signaling pathway. However, the inhibition of the expression of αS1-casein by 17.5 and 21 mM glucose may be associated with the changes of key enzymes' activity involved in glucose metabolism.