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Mechanism of activation of lipolysis in Manduca sexta: Role of triglyceride-lipase

Patel, Rajesh Tulsibhai
Scope and Method of Study:
Insects are the largest group of organism on the planet. Because of economical and ecological reasons, it is important to understand insects. The main aim of this study was to advance the knowledge of lipid metabolism using Manduca sexta as a model organism. In vivo and in vitro experiments were used to investigate the intracellular signaling mechanisms involved in the activation of triglyceride (TG) lipase by the lipid mobilizing hormone, proteins involved in the TG hydrolysis, and characterization of TG-lipase.
Findings and Conclusion:
Our results identified that adipokinetic hormone (AKH) stimulates lipolysis by highly phosphorylating "Lipid Storage Droplet Protein 1" (Lsdp1) of the lipid droplets and increase in TG-lipase activity of cytosol. The phosphorylation of Lsdp1 is important for lipolysis and is mediated by cAMP dependent protein kinase (PKA). The changes in the phosphorylation of Lsdp1 correlated with the increase in TG-lipase activity suggesting that this protein is a major player in the activation of lipolysis. Although AKH increases TG-lipase activity of cytosol isolated from hormone stimulated insects compare to control insects, it does not change the state of phosphorylation of TG-lipase, indicating that phosphorylation of the TG-lipase plays no role in the activation of lipolysis. Overall, the changes in the lipid droplets are responsible for ~70% of the lipolytic response to AKH. The remaining 30% appears to be due to AKH-dependent changes in cytosol. We further identified that TG-lipase has phospholipase A1 activity and is able to hydrolyze phospholipids present on the surface of the lipid droplets. Altogether, it is concluded that lipolysis in fat body adipocyte is mainly controlled by activation of the substrate.