Insulin regulation of lipoprotein lipase (LPL) activity and expression in gilthead sea bream (Sparus aurata)

Lipoprotein lipase (LPL) is a key enzyme in lipoprotein metabolism by virtue of its capacity to hydrolyze triglycerides circulating in the form of lipoprotein particles. Here we analyzed the fasting effects of LPL in gilthead sea bream (Sparus aurata) and also present the first study in fish of the role of insulin as a potential modulator of both LPL activity and expression. Fasting for 2 weeks provoked a clear decrease in adipose tissue LPL activity, concomitant with lower levels of plasma insulin, while no effects were observed in red muscle. To elucidate the specific role of insulin, increases of plasma insulin were experimentally induced by arginine and insulin injections. However, arginine predominantly stimulated glucagon over insulin secretion in this fish species while LPL activity did not change significantly in adipose tissue. Instead, insulin administration induced an increase in adipose tissue LPL activity 3 h after the injection, whereas LPL activity in red muscle was not affected. Changes in LPL activity were accompanied by an increase in LPL mRNA levels in the adipose tissue of insulin-injected gilthead sea bream, although changes in LPL expression were delayed in time with respect to variations in LPL activity. Finally, LPL mRNA levels in red muscle were similar between control and insulin-injected gilthead sea bream, suggesting that insulin does not play a direct role in the regulation of LPL in this tissue. The current study shows that LPL activity is regulated by nutritional condition and underscores the importance of insulin as a modulator of LPL activity and expression in the adipose tissue of gilthead sea bream.