Lipid-lowering effect of PPARα activated by feed EPA and ALA in GIFT Oreochromis niloticus

Dietary fat is an essential nutrient for fish growth, yet the impact of PPARα activation by fatty acids on lipid metabolism in fish is not well understood. This study aimed to investigate the role of PPARα activation and its regulatory effects on lipid metabolism in response to fatty acids in GIFT Oreochromis niloticus using both in vitro and in vivo approaches. Primary hepatocytes from tilapia were incubated with seven individual fatty acids: oleic acid (OA), palmitic acid (PA), alpha-linolenic acid (ALA), linoleic acid (LA), arachidonic acid (AA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), and lipid metabolism were measured. ALA, AA and EPA significantly up-regulated the expressions of key fatty acid catabolic genes such as PPARα, carnitine palmitoyltransferase 1 (CPT1) and acyl-CoA oxidase (ACO). ALA specifically up-regulated the expressions of lipolysis related genes adipose triglyceride lipase (ATGL), adipose triglyceride lipase (HSL) and fatty acid transport-related genes adipose triglyceride lipase (LPL), respectively. EPA inhibited the expression of ACC, and ALA, EPA and AA could reduce the content of triglycerides in cells. In contrast, PA and OA increased triglyceride content and the number of lipid droplets. In vivo experiments were conducted with two fatty acids, EPA and ALA, which showed distinct gene activation patterns. Six semi-purified diets varying in ALA (0.6%, 1.2%) and EPA (0.6%, 1.2%) were formulated for normal fat (7%) group, high fat (12%) group (HF) and high fat group with EPA or ALA (0.6%, 1.2%), respectively. Tilapia with an initial body weight of (3.53±0.03) g were fed these diets for eight weeks. No significant differences in growth were observed among the groups. However, body fat was significantly reduced in all four treatment groups compared to the high-fat group. As ALA and EPA content increased, the condition factor (CF), hepatosomatic index (HSI), and muscle fat index (MFI) decreased, along with serum and liver triglycerides (TG) and cholesterol. No significant differences were found in levels of aspartate aminotransferase (AST), alanine aminotransferase (ALT), superoxide dismutase (SOD), and malondialdehyde (MDA) among the groups. Notably, MDA in the liver was significantly increased in the EPA (1.2%) group, and the expression of PPARα, CPT1, ACO, ATGL and HSL was significantly enhanced with EPA and ALA supplementation. Collectively, these findings indicate that diets containing 0.6% EPA or ALA can up-regulate the expression of PPARα and its lipid metabolism-related target genes, leading to a lipid lowering effect. The study's outcomes offer valuable insights for feed formulation and fat source screening.