Lipid-induced lipotoxic damage in liver and hepatocytes of black seabream Acanthopagrus schlegelii is mediated by endoplasmic reticulum stress

Liver is considered the critical tissue for metabolic processing of dietary nutrients including lipids. High-fat diets (HFD) can induce hepatic lipotoxic injury, but the mechanism is unclear. This study aimed to elucidate the underlying role of endoplasmic reticulum stress (ERS) in mediating HFD-induced hepatic lipotoxic injury in black seabream (Acanthopagrus schlegelii) via in vitro and in vivo experiments. For the in vitro experiment, isolated hepatocytes from black seabream were incubated for 24 h in medium (Control), or medium supplemented with 200 μM oleic acid (OA) alone or in combination with 20 μM 4-phenyl butyric acid (OA + 4-PBA). In the in vivo experiment, black seabream juveniles (3.30 ± 0.10 g) were fed diets containing lipid at 12.4% (Control) or 18.6% (HFD) for 8 weeks, after which the HFD-fed fish were divided into three groups and, on alternate days over a period of 8 days, subjected to intraperitoneal injections of 50 mg/kg 4-PBA in hydroxypropyl-β-cyclodextrin (HBC) (treatment 4-PBA) or carrier alone (treatment HFD + HBC) versus no injection (HFD). The contents of triglyceride and non-esterified fatty acid, as well as expression levels of genes and proteins involved in lipid metabolism, ERS, apoptosis and inflammation were determined in liver and isolated hepatocytes, and oil-red O staining, ultrastructural observation, flow cytometry assay of apoptosis rate, and terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling assay (TUNEL) staining were performed on hepatocytes. The in vitro experiment showed that 4-PBA treatment significantly decreased lipid accumulation, and the OA-induced expression levels of genes/proteins linked to ERS, lipogenesis, inflammation and apoptosis in hepatocytes. Transmission Electron Microscope (TEM) imaging showed that 4-PBA treatment mitigated the ER swelling induced by OA. These results were confirmed in the in vivo experiment, where 4-PBA injection reduced lipid deposition and expression levels of genes/proteins related to ERS, lipogenesis, inflammation and apoptosis in liver compared to the HFD treatment. In conclusion, these results confirmed that ERS plays a contributing regulatory role in HFD- and OA-induced lipotoxic damage including lipid accumulation, apoptosis and inflammation in liver and isolated hepatocytes of black seabream, which supply a new perspective in the cross-talk of ERS and hepatic lipotoxic damage in hepatocytes/liver of marine fish and provide theoretical guidance for the prevention and treatment of hepatic lipotoxic damage.