Effects of insulin and its related signaling pathways on lipid metabolism in the yellow catfish Pelteobagrus fulvidraco

The influence of insulin on hepatic metabolism in fish is not well understood. The present study was therefore conducted to investigate the effects of insulin on lipid metabolism, and the related signaling pathways, in the yellow catfish Pelteobagrus fulvidraco. Hepatic lipid and intracellular triglyceride (TG) content, the activity and expression levels of several enzymes and the mRNA expression of transcription factors (PPAR alpha and PPAR gamma) involved in lipid metabolism were determined. Troglitazone, GW6471, fenofibrate and wortmannin were used to explore the signaling pathways by which insulin influences lipid metabolism. Insulin tended to increase hepatic lipid accumulation, the activity of lipogenic enzymes (6PGD, G6PD, ME, ICDH and FAS) and mRNA levels of FAS, G6PD, 6PGD, CPT IA and PPAR., but down-regulated PPAR alpha mRNA level. The insulin-induced effect could be stimulated by the specific PPAR gamma activator troglitazone or reversed by the PI3 kinase/Akt inhibitor wortmannin, demonstrating that signaling pathways of PPAR gamma and PI3 kinase/Akt were involved in the insulin-induced alteration of lipid metabolism. The specific PPAR alpha pathway activator fenofibrate reduced insulin-induced TG accumulation, down-regulated the mRNA levels of FAS, G6PD and 6PGD, and up-regulated mRNA levels of CPT IA, PPAR alpha and PPAR gamma. The specific PPAR alpha pathway inhibitor GW6471 reduced insulin-induced changes in the expression of all the tested genes, indicating that PPAR alpha mediated the insulin-induced changes of lipid metabolism. The present results contribute new knowledge on the regulatory role of insulin in hepatic metabolism in fish.