Single and combined effects of high-fat diet and ammonia nitrogen on oxidative stress, lipid accumulation and inflammation in liver of grass carp Ctenopharyngodon idella

The current study aims to investigate the role of high fat diet (HFD) and ammonia nitrogen (AN) in the fatty liver disease progression in farmed fish. In the present study, grass carp were fed the normal fat diet or HFD for 4 weeks to set up the Control group, HFD group, AN group and HFD + AN group. In the AN group and HFD + AN group, grass carp were exposed to 0.52 mg/L NH3-N for the last week. The results show that HFD feeding had no effect on the growth of grass carp but AN stress significantly inhibited it. Further, the HFD feeding and AN challenge both significantly increased the level of alanine aminotransferase (ALT) and reduced the total antioxidant capacity (T-AOC) significantly. Moreover, a further aggravated liver injury appeared in HFD-fed grass carp after AN challenge, as evidenced by the significant increasing of the level of ALT, AST and malondialdehyde (MDA) and marked reducing of glutathione (GSH) and T-AOC. Moreover, the results of Oil Red O and TG contents in liver all indicate that HFD and AN challenge both promote the lipid accumulation in grass carp liver. Corresponding to this, the results of genes and protein related to lipid metabolism reveal that AN challenge dramatically inhibited lipid hydrolysis, lipophagy and fatty acid oxidation in grass carp liver but the single HFD feeding and AN challenge did not have this effect. Moreover, the single HFD feeding and AN challenge both promoted fatty acid transport to the liver and lipid synthesis in grass carp liver. In summary, the HFD feeding and AN challenge both can disturb the lipid metabolism in grass carp liver. In parallel, the results of expression levels of genes and protein related to inflammation indicated that the single HFD feeding may not significantly contribute to the inflammation in grass carp liver, but the single AN challenge apparently promoted the inflammatory process. Most importantly, inflammatory progression in liver was further aggravated in the HFD-fed grass carp after AN challenge compared to grass carp with only AN stress, mostly evidenced by the increased levels of inflammatory factors (il1β, il2, TNF-α, etc.). In summary, AN challenge more significantly promoted the lipid accumulation and inflammatory process in the liver of HFD-fed grass carp, which may have further aggravated liver injury.