Regulation of lipid metabolism in grass carp hepatocytes by exosomes derived from fatty hepatocytes though GRP78

Exosomes regulate lipid metabolism by carrying miRNAs, nucleic acids, and proteins, thereby influencing the function of receptor cells. Glucose-regulated protein 78 (GRP78) is also involved in the regulation of lipid metabolism. However, it remains unclear whether exosomes derived from fatty hepatocytes (OA-Exo) regulate lipid metabolism through the enrichment of GRP78. In this study, we observed lipid accumulation after incubating hepatocytes with oleic acid (OA) for 24 hours (fatty hepatocytes) ( P  < 0.05). Interestingly, the expression of GRP78 was significantly increased in fatty hepatocytes and OA-Exo ( P  < 0.05). We hypothesized that GRP78 plays an important role in the regulation of lipid metabolism. Subsequently, the hepatocytes were separately incubated with OA-Exo (50 µg/mL) and GRP78 protein (1 µg/mL) for 24 hours. The results showed a significant increase in the content of Triacylglycerol (TG) and total cholesterol (TC), as well as up-regulation of the expression of GRP78 and inositol-requiring enzyme-1alpha (IRE1α) protein ( P  < 0.05). It suggested that exosomal GRP78 promoted lipid accumulation in hepatocytes. Therefore, we used YUM70 (an inhibitor of GRP78) to inhibit endogenous GRP78, followed by incubation of hepatocytes with OA-Exo and GRP78 for 24 hours. Compared with the control group, the content of TG and the expression of GRP78 protein were significantly decreased in the YUM70 group ( P  < 0.05). However, when compared with the YUM70 group, OA-Exo reversed the effect of YUM70 and increased the content of TG, TC, and the expression of GRP78 protein in hepatocytes ( P  < 0.05). These results suggested that OA-Exo promoted lipid accumulation in hepatocytes through the recruitment of GRP78, and the IRE1α pathway may be important in this process. Furthermore, the inhibition of the IRE1α pathway with 4µ8C resulted in a significant decrease in TG content compared to the control group ( P  < 0.05). However, when compared with the 4µ8C group, OA-Exo and GRP78 reversed the effect of 4µ8C and significantly increased TG content ( P  < 0.05). Taken together, these results indicated that OA-Exo activated IRE1α to promote lipid accumulation in hepatocytes through the enrichment of GRP78. This study provided a new perspective for further exploration of exosomal lipid metabolism in fish.