Effects of dietary plant protein sources influencing hepatic lipid metabolism and hepatocyte apoptosis in hybrid grouper (Epinephelus lanceolatus♂ × Epinephelus fuscoguttatus♀)

A 56-day feeding trial was conducted to evaluate the effects of replacing dietary fish meal (FM) with a complementary mixture of plant protein (PP) consisting of soybean meal (SBM), corn gluten meal (CGM) and cottonseed meal (CSM) on hepatic lipid metabolism and hepatocyte apoptosis in juvenile hybrid grouper, Epinephelus lanceolatus♂ × E. fuscoguttatus♀ (31.64 ± 0.82 g). A basal diet (FM60) with FM as the sole protein source was compared to diets progressively replacing 25% (FM45), 50% (FM30) and 75% (FM15) of FM protein. No significant differences were observed in growth performance and feed utilization when up to 75% of FM protein was replaced by PP sources. The hepatosomatic index (HSI) was markedly increased as dietary PP inclusion increased, but crude lipid content in the liver showed the opposite trend. Plasma cholesterol (CHO), high-density lipoprotein cholesterol (HDL-C) and low-density lipoprotein cholesterol (LDL-C) of fish fed the PP containing diets were all significantly lower than the control. Liver histological analysis showed a higher levels of hepatocyte vacuolization and nuclear pycnosis occurred as dietary PP inclusion increased, but the amounts of hepatic lipid droplets showed a decreasing trend with increasing PP inclusion levels. Moreover, dietary PP inclusion down-regulated the mRNA levels of lipid metabolism-related genes including peroxisome proliferator-activated receptor alpha (PPARα), carnitine palmitoyltransferase 1 (CPT1), apolipoprotein AI (Apo-AI) and lipoprotein lipase (LPL). On the other hand, dietary PP sources also down-regulated the apoptosis-related genes including caspase-3, caspase-7, caspase-8 and p53. The present study provided new evidence for the PP sources-induced lipid metabolism in carnivorous fish, and provided new insight into the relationship between dietary PP sources and cell apoptosis.