Conjugated linoleic acid differentially modulates growth, tissue lipid deposition, and gene expression involved in the lipid metabolism of grass carp

• No effective treatment has been developed to relieve fatty liver syndrome of fish
• The molecular process involved in fat reduction effects of CLA in fish remain unclear
• 3% CLA in grass carp diets induced a reduced growth rate
• Liver lipid contents were significantly reduced in the 2.5% and 3% CLA groups
• Dose- and tissue-dependent changes were found in the mRNA expressions in grass carp

Conjugated linoleic acid (CLA) has been shown to decrease body fat and increase lean tissue in mammals. However, limited data is available about the effect of CLA on the lipid content in fish tissue, and the mechanisms underlying the beneficial effects of CLA in fish are unknown. We hypothesized that dietary CLA may induce lipid-lowering effects in grass carp (Ctenopharyngodon idella) tissue, and the fat reduction effect was modulated by the expression of genes involved in the lipid metabolism. A 65-day growth trial was conducted to investigate the effect of CLA on the growth, tissue lipid deposition, and gene expression involved in the lipid metabolism of grass carp. Seven isonitrogenous and isolipidic diets were formulated: 0% CLA (control); 0.5% CLA (CLA0.5); 1% CLA (CLA1); 1.5% CLA (CLA1.5); 2% CLA (CLA2); 2.5% CLA (CLA2.5); and 3% CLA (CLA3).Results showed that only fish fed the CLA3 diet exhibited a significant reduction in feeding rate and specific growth rate than those of fish fed the control diet (P < 0.05). Significant decreases in the lipid content in the liver, intraperitoneal fat, and muscle were observed in fish fed with 2.5% to 3% CLA, 1.5% to 3% CLA, and 2% to 3% CLA diets, respectively (P < 0.05), compared to those fed with the control diet. Dose- and tissue-dependent changes were found in the mRNA expressions of fatty acid synthetase (FAS), acetyl-CoA carboxylase (ACC), lipoprotein lipase (LPL), hormone-sensitive lipase (HSL), peroxisome proliferator-activated receptor α (PPARα), peroxisome proliferator-activated receptor γ (PPARγ), and sterol regulatory element binding protein-1c (SREBP-1c). The mRNA expressions in most of the genes examined in the liver, foregut, intraperitoneal fat and muscle were highly sensitive to dietary CLA. Our results suggested that a dose-dependent effect on the reduction of fish growth induced by CLA supplementation should be carefully considered in intensive aquaculture, although lipid reduction is nutritionally important for fatty liver control in grass carp. Furthermore, our results raise the possibility that the lipid-lowering effects of dietary CLA were modulated by the gene expressions in lipogenesis (such as FAS and ACC), lipoprotein transport (such as LPL), and lipolysis (such as HSL) primarily in the liver, foregut, intraperitoneal fat, and muscle. The activation of transcription factors (such as PPARα, PPARγ, and SREBP-1c) may also be responsible for the lipid-lowering effects of dietary CLA.