Abstract
Exploring exogenous additives to enhance non-protein energy ingredient utilization in fish is highly anticipated. Epigallocatechin-3-gallate (EGCG) is a prominent polyphenol found in green tea. To assess the regulatory effect of EGCG on metabolic disorders induced by an excess of non-protein energy sources in grass carp (Ctenopharyngodon idella), five isonitrogenous diets were formulated: a control diet (23% digestible carbohydrates, 4.7% crude lipids), a high-carbohydrate (HC; 48% digestible carbohydrates, 4.7% crude lipids) diet, a high-fat (HF; 23% digestible carbohydrates, 9.6% crude lipid) diet, the HC diet supplemented with 0.05% EGCG (HC + EGCG) and the HF diet supplemented with 0.05% EGCG (HF + EGCG). The diets were fed to 450 fish (initial body weight = 8.50 g) in three replicates for eight weeks. After the feeding trial, compared with the control group, EGCG significantly alleviated the growth suppression induced by HC or HF intake (P < 0.05). Dietary EGCG supplementation downregulated the expression of fatty acid synthesis genes (srebp1, acc, and fas) (P < 0.05). Further, EGCG supplementation to the HC diet significantly increased hepatic glycogen synthesis and suppressed elevated glucose concentration through gluconeogenesis (P < 0.05). Additionally, EGCG supplementation significantly reduced excess circulating glucose by inhibiting α-amylase activities and decreasing the expression of glucose transporter 2 (glut2) in the intestines of grass carp (P < 0.05). Furthermore, in order to reveal the mechanism of EGCG affecting feeding and metabolism, transcriptome analysis on grass carp hepatocytes was conducted. The results showed that the leptin b (lepb) and G protein-coupled bile acid receptor 1 (gpbar1) were key candidate genes among differentially expressed genes from the cell transcriptome. Further in vivo studies found that the high leptin release induced by EGCG may play a pivotal role in the regulation of feed intake and appetite. Moreover, through in silico docking and microscale thermophoresis analysis, G protein-coupled bile acid receptor 1 (GPBAR1) was identified as a potential membrane receptor that directly interacts with EGCG, mediating its regulatory effect on metabolism. Therefore, plant-derived products containing high levels of EGCG could be considered as natural feed additives for regulating glucose redistribution, as they can mitigate the elevated blood glucose levels in fish caused by consuming a HC diet.