Abstract
Farmed fish often suffers from fatty liver, which is characterized by the aberrant lipid accumulation. Lipid droplet is an intracellular organelle to store neutral lipid, and the formation of excessive lipid droplets in hepatocytes is associated with fatty liver disease. However, in the present study, we found that 2-week high fat (lipid 8%) intake have no adverse effect on grass carp hepatopancreas compared with control group (lipid 4%) accompanied by the lipid accumulation, indicating that the formation of lipid droplets may be a protection mechanism in response to exogenous fatty acids in farmed fish. Then, highly selective small-molecule inhibitors of DGAT (diacylglycerol acyltransferase) and oleic acid were combined to verify this hypothesis in grass carp hepatocytes. We found that inhibition of DGAT1 markedly abolished formation of lipid droplets and had a strong effect on TG (triglyceride) levels, while inhibition of DGAT2 decreased the formation of large lipid droplets and had no effect on TG levels, suggesting that DGAT1 is responsible for lipid droplet formation but DGAT2 only promotes the fusion of lipid droplet produced by DGAT1 in grass carp hepatocytes. Moreover, inhibition of DGAT1 not DGAT2 attenuated expression of lipogenic genes, which may contribute to the differential effects of DGAT1 and DGAT2 on TG synthesis. We demonstrate that inactivation of DGAT1 not DGAT2 set a metabolic switch towards fat oxidation that increased the content of reactive oxygen species, thereby leading to hepatic lipotoxicity via inducing endoplasmic reticulum stress/apotosis and inflammation. Taken together, DGAT1-dependent lipid droplet biogenesis is the cellular protective response to exogenous fatty acids in the development of hepatic steatosis. The results will help us to understand the physiological significance of lipid droplet in fish hepatocytes and the mechanisms of progressive development of farmed fish fatty liver
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