Effects of Dietary Lysine Levels on Growth Performance and Glycolipid Metabolism via the AKT/FoxO1 Pathway in Juvenile Largemouth Bass, Micropterus salmoides

Abstract

A 56-day feeding experiment was conducted to determine the dietary lysine requirement of juvenile largemouth bass (Micropterus salmoides) and investigate the effects of dietary lysine on growth, whole-body composition, and hepatic gene expression related to glycolipid metabolism via the AKT/FoxO1 pathway. The juveniles ( $17.34\pm 0.02$  g) were fed six graded lysine levels (2.11% (control), 2.56%, 2.92%, 3.33%, 3.68%, and 4.09%, dry diet). The results showed that the 3.33% dietary lysine level significantly increased the final body weight (FBW), weight gain rate (WGR), and specific growth rate (SGR) and improved the feed conversion ratio (FCR) compared with the control group. The whole-body composition was not significantly affected by dietary lysine levels, while lowest hepatic lipid contents were found in the 2.92% and 3.33% dietary lysine groups. Regarding glycolipid metabolism, compared with the control group, 3.33% dietary lysine improved the protein kinase B (AKT) and inhibited the forkhead box O1 (FoxO1), thus upregulated the pyruvate kinase (PK) mRNA levels to enhance glycolysis. Furthermore, sterol-regulatory element binding protein-1c (SREBP1c) and peroxisome proliferator-activated receptor-γ (PPAR-γ) were downregulated by 3.33% dietary lysine, which caused the downregulation of lipid synthesis-related genes acetyl-CoA carboxylase-1 (ACC) and stearyl-CoA desaturase (SCD) mRNA. In addition, 3.33% dietary lysine promoted the expression of the lipolysis-related genes peroxisome proliferator-activated receptor-α (PPAR-α) and carnitine palmitoyl transferase-1 (CPT1). According to the quadratic regression analysis based on the FCR and SGR values, the optimal dietary lysine levels were estimated to be 3.03% and 3.07% of the diet (6.39% and 6.48% of dietary protein), respectively.