Effect and the related signaling pathways of insulin influencing lipid metabolism in yellow catfishPelteobagrus fulvidraco

Abstract

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 (PPARα and PPARγ) 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 PPARγ, but down-regulated PPARα mRNA level. The insulin-induced effect could be stimulated by the specific PPARγ activator troglitazone or reversed by the PI3 kinase/Akt inhibitor wortmannin, demonstrating that signaling pathways of PPARγ and PI3 kinase/Akt were involved in the insulin-induced alteration of lipid metabolism. The specific PPARα pathway activator fenofibrate reduced insulininduced TG accumulation, down-regulated the mRNA levels of FAS, G6PD and 6PGD, and up-regulated mRNA levels of CPT IA, PPARα and PPARγ. The specific PPARα pathway inhibitor GW6471 reduced insulin-induced changes in the expression of all the tested genes, indicating that PPARα 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.

AKTs/PKBs: molecular characterization, tissue expression and transcriptional responses to insulin and/or wortmannin in yellow catfish Pelteobagrus fulvidraco

“…Bouraoui et al (2010) found that AKT seemed to be involved in the stimulation of glucose uptake by insulin in rainbow trout adipocytes. Recently, by using the specific inhibitor wortmannin of PI3K/AKT pathway, insulininduced liver lipid accumulation is significantly reduced in yellow catfish (Zhuo et al 2015). Since AKT is the main downstream enzyme of PI3K, it is interesting to explore which AKT isoform may respond to the insulin and/or wortmannin in yellow catfish.…”

“…Especially, Bouraoui et al (2010) found that the AKT pathway was involved in the insulin-induced effects of trout adipocytes during the various stages of adipogenesis. Recently, we found that insulin stimulated liver lipid accumulation in yellow catfish (Zhuo et al 2014), and insulin-induced liver lipid accumulation are significantly reduced by using the specific inhibitor wortmannin of PI3K (Zhuo et al 2015). However, in that study, Zhuo et al (2015) did not elucidate whether insulin affected lipid metabolism through the PI3K/AKT pathway or which AKT isoform involved insulin signal pathway.…”

“…Recently, we found that insulin stimulated liver lipid accumulation in yellow catfish (Zhuo et al 2014), and insulin-induced liver lipid accumulation are significantly reduced by using the specific inhibitor wortmannin of PI3K (Zhuo et al 2015). However, in that study, Zhuo et al (2015) did not elucidate whether insulin affected lipid metabolism through the PI3K/AKT pathway or which AKT isoform involved insulin signal pathway. Furthermore, studies suggested that different AKT isoforms played distinct physiological roles (Cho et al 2001, George et al 2004Easton et al 2005).…”

“…Prior to the injection, they fasted for 24 h. Then, they were anesthetized (MS-222 at 100 mg/L) and intraperitoneally injected with 1 μL/g fish body weight (BW) of PBS (control) or with different doses of insulin (0.01, 0.1 and 1 μg/g BW, respectively), and the concentration of insulin was selected according to our previous study (Zhuo et al 2015). After injection, the fish were returned to their corresponding tanks.…”

“…The hepatocyte cell suspension (CS) was plated onto 25-cm 2 flasks at the density of 1 × 10 6 cells/ mL. The hepatocytes then were incubated with 10 mM PBS (control), 1 μM wortmannin, 100 nM insulin and 1 μM wortmannin +100 nM insulin, and the concentrations of insulin and wortmannin were selected according to our previous studies (Zhuo et al 2014;Zhuo et al 2015). Each treatment was performed in triplicate and three independent experiments were carried out.…”