Insulin Stimulates Lipogenesis and Attenuates Beta-Oxidation in White Adipose Tissue of Fed Rainbow Trout

As lipid deposition tissue in fish, the white adipose tissue (WAT) has important functions related to reproduction and the challenges of long-term fasting. In the study reported here, we infused fish fed a high-carbohydrate diet with two doses of insulin for 5 days in order to explore the effects of this hormone on lipogenesis and beta-oxidation-related enzymes. We demonstrated the presence of some of the main lipogenic enzymes at molecular, protein and activity levels (ATP-citrate lyase and fatty acid synthase). However, while ATP-citrate lyase was unexpectedly down-regulated, fatty acid synthase was up-regulated (at protein and activity levels) in an insulin dose-dependent manner. The main enzymes acting as NADPH donors for lipogenesis were also characterized at biochemical and molecular levels, although there was no evidence of their regulation by insulin. On the other hand, lipid oxidation potential was found in this tissue through the measurement of gene expression of enzymes involved in beta-oxidation, highlighting two carnitine palmitoyltransferase isoforms, both down-regulated by insulin infusion. We found that insulin acts as an important regulator of trout WAT lipid metabolism, inducing the final stage of lipogenesis at molecular, protein and enzyme activity levels and suppressing beta-oxidation at least at a molecular level. These results suggest that WAT in fish may have a role that is important not only as a lipid deposition tissue but also as a lipogenic organ (with possible involvement in glucose homeostasis) that could also be able to utilize the lipids stored as a local energy source.