The ameliorative role of methionine in hepatic steatosis and stress response in juvenile black seabream (Acanthopagrus schlegelii) fed with a high-fat diet

Methionine (Met) is not only an essential amino acid of aquatic animals, but it also regulates lipid metabolism. To investigate the impacts of dietary Met supplementation on lipid metabolism, oxidative stress (OS), inflammation and apoptosis in marine fish, a feeding trial lasting for eight weeks was conducted on juvenile black seabream ( Acanthopagrus schlegelii ) (initial weight 1.61 ± 0.01 g) fed with five different experimental diets: Control, regular fat diet (11% lipid level); HFD, high-fat diet (17% lipid level), HFD contained different contents of Met (5, 10 or 20 g/ kg) named HFD + M1, HFD + M2, and HFD + M3, respectively. Serum biochemical indices results indicated that the contents of triglyceride (TG), total cholesterol (TC), non-esterified fatty acid (NEFA) and low-density lipoprotein cholesterol (LDL-C), as well as the activity of alanine transaminase (ALT), markedly decreased in HFD + M1 group compared to those in the HFD treatment. Results of liver sections indicated that lipid droplets dramatically decreased by dietary Met supplementation than the HFD group. Similar results were reflected in transmission electron microscope (TEM) images. The mRNA-abundant levels of adenosine monophosphate-activated protein kinase α ( ampkα ) and silent information regulator 1 ( sirt1 ) significantly increased by dietary HFD + M1 than those in the HFD treatment. When comparing to the HFD treatment, the levels of gene expression for lipogenesis pathway genes, specifically sterol regulatory element-binding protein-1c ( srebp-1c ) and fatty acid synthase ( fas ) were markedly improved by dietary HFD + M1. In contrast, opposite results were recorded in the mRNA expression levels of lipolysis pathway related genes ( pparα , cpt1a , hsl , atgl , lpl ) that were down-regulated by the HFD + M1 by promoting related lipogenesis. The transcriptional expression level of srebp-1c was significantly suppressed in HFD + M1 diet. HFD caused oxidative stress, inflammation and apoptosis in the liver reflected in increasing the reactive oxygen species (ROS) and malonaldehyde (MDA) contents, c-Jun N-terminal kinase ( jnk ) and nuclear factor kappa B ( nf-κb ) mRNA expression levels, but those physiological stresses could be alleviated by dietary Met supplementation. Specifically, the antioxidant enzyme activities and gene expression levels were notably up-regulated by HFD + M1 group than those in fish fed with HFD. Additionally, compared to the HFD treatment, dietary Met supplementation significantly decreased the transcriptional expression levels of pro-inflammation and pro apoptosis genes, but up-regulated the transcriptional expression of anti-inflammatory cytokine and anti-apoptosis gene. Together, this study elucidates that HFD supplemented with Met can ameliorate hepatic steatosis, oxidative stress, inflammation and apoptosis induced by HFD, confirming that Met can play a vital role in lowing lipid accumulation by regulating lipid metabolism in A. schlegelii fed with HFD.