Leucine Affects α-Amylase Synthesis through PI3K/Akt-mTOR Signaling Pathways in Pancreatic Acinar Cells of Dairy Calves

Dietary nutrient utilization, particularly starch, is potentially limited by digestion in dairy cow small intestine because of shortage of alpha-amylase. Leucine acts as an effective signal molecular in the mTOR signaling pathway, which regulates a series of biological processes, especially protein synthesis. It has been reported that leucine could affect alpha-amylase synthesis and secretion in ruminant pancreas, but mechanisms have not been elaborated. In this study, pancreatic acinar (PA) cells were used as a model to determine the cellular signal of leucine influence on alpha-amylase synthesis. PA cells were isolated from newborn Holstein dairy bull calves and cultured in Dulbecco's modifed Eagle's medium/nutrient mixture F12 liquid media containing four leucine treatments (0, 0.23, 0.45, and 0.90 mM, respectively), following alpha-amylase activity, zymogen granule, and signal pathway factor expression detection. Rapamycin, a specific inhibitor of mTOR, was also applied to PA cells. Results showed that leucine increased (p < 0.05) synthesis of alpha-amylase as well as phosphorylation of PI3K, Akt, mTOR, and S6K1 while reduced (p < 0.05) GCN2 expression. Inhibition of mTOR signaling downregulated the alpha-amylase synthesis. In addition, the extracellular leucine dosage significantly influenced intracellular metabolism of isoleucine (p < 0.05). Overall, leucine regulates alpha-amylase synthesis through promoting the PI3K/Akt-mTOR pathway and reducing the GCN2 pathway in PA cells of dairy calves. These pathways form the signaling network that controls the protein synthesis and metabolism. It would be of great interest in future studies to explore the function of leucine in ruminant nutrition.