Short communication: Relationship between lysine/methionine ratios and glucose levels and their effects on casein synthesis via activation of the mechanistic target of rapamycin signaling pathway in bovine mammary epithelial cells

The synthesis of protein requires the availability of specific AA and a large supply of energy in bovine mammary epithelial cells (BMEC). Whether an interaction exists between Lys/Met ratio and glucose level on milk protein synthesis and its potential regulatory mechanism is unclear. We investigated the effects of different Lys/Met ratios and glucose levels on casein synthesis-related gene expression in BMEC to elucidate the underlying molecular mechanisms. Primary BMEC were subjected to 4 treatments for 36 h, arranged in a 2 × 2 factorial design with Lys/Met ratios of 3:1 (1.2:0.4 mM, LM3.0; total AA = 8.24 mM) and 2.3:1 (1.4:0.6 mM, LM2.3; total AA = 8.64 mM) and glucose levels of 17.5 mM (high glucose level) and 2.5 mM (low glucose level). No interactions between Lys/Met ratio and glucose level on cell viability, cell cycle progression, mRNA, or protein expression levels were found. High glucose level increased cell proliferation and promoted cell cycle transition from intermediate phase (G1 phase) to synthesis (S phase) by approximately 50%, whereas Lys/Met ratio had no effect. Both mRNA and protein abundance of αS₁-casein and β-casein were positively affected by LM3.0, whereas a high glucose level increased protein abundance of αS₁-casein and β-casein and increased gene expression of CSN1S1 but not of CSN2. Furthermore, high glucose increased the mRNA abundance of ELF5 and decreased that of GLUT8, enhanced protein expression of total and phosphorylated mechanistic target of rapamycin (mTOR), and decreased phosphorylated AMP-activated protein kinase (AMPK) levels. Treatment LM3.0 had a stimulatory effect on total and phosphorylated mTOR but did not affect AMPK phosphorylation. The mRNA levels of JAK2, ELF5, and RPS6KB1 were upregulated and mRNA levels of EIF4EBP1 were downregulated with LM3.0 compared with LM2.3. Our results indicate that casein synthesis was regulated by Lys/Met ratio via JAK2/ELF5, mTOR, and its downstream RPS6KB1 and EIF4EBP1 signaling. In contrast, glucose regulated casein synthesis through promoting cell proliferation, accelerating cell cycle progression, and activating the ELF5 and AMPK/mTOR signaling pathways. Within the range of substrate levels in the present study, a change in Lys/Met ratio had a stronger effect on abundance of αS₁-casein and β-casein than a change in glucose level.