Activation of the Lipid Droplet Controls the Rate of Lipolysis of Triglycerides in the Insect Fat Body. A role for Lsdp1 (Lipid storage droplet protein 1)

Abstract

The hydrolysis of triglyceride (TG) stored in the lipid droplets (LD) of the insect fat body is under hormonal regulation by the adipokinetic hormone (AKH), which triggers a rapid activation of cAMP-dependent kinase cascade (PKA). The role of phosphorylation on two components of the lipolytic process, the TG-lipase and the LD was investigated. The activity of purified TG-lipase determined using in vivo TG-radiolabeled LD was unaffected by the phosphorylation of the lipase. However, the activity of purified lipase was 2.4-fold higher against LD isolated from AKH-stimulated fat bodies than unstimulated tissue. Investigation of the phosphorylation state of LD- associated proteins showed that in vivo stimulation of lipolysis promotes a rapid phosphorylation of a 42-44kDa protein. This protein was identified by mass spectrometry as Lsdp1. In vivo phosphorylation of this protein reached a peak ~10min after the injection of AKH. Supporting a role of Lsdp1 in lipolysis, maximum TG-lipase activity was also observed with LD isolated 10min after hormonal stimulation. The activation of lipolysis was reconstituted in vitro using purified insect PKA and TG-lipase, and LD, without additional cellular components. In vitro phosphorylation of LD catalyzed by PKA enhanced the phosphorylation of Lsdp1 and the lipolytic rate of the lipase. It is concluded that the lipolysis in fat body adipocytes is controlled by the activation of the LD. Lsdp1 is the main targetof PKA, suggesting that this protein is a major player in theactivation of lipolysis in insects. Funding provided by OAES and NIH.