Triglycerides (TG) stores build up in the insect fat body as lipid droplets at times of excess of food. The mobilization of fat body triglyceride (TG) is stimulated by adipokinetic hormones (AKH). The action of AKH involves a rapid activation of cAMP-dependent protein kinase (PKA). Recent in vitro studies have shown that PKA phosphorylates and activates the TG-lipase substrate, the lipid droplets. Conversely, purified TG-lipase from Manduca sexta fat body is phosphorylated by PKA in vitro but is not activated. This study was directed to learn whether or not AKH promotes a change in the state of phosphorylation of the lipase in vivo, and what are the relative contributions of cytosol and lipid droplets to the overall increase of lipolysis triggered by AKH. TG-lipase activity of fat body cytosols isolated from control and AKH-treated insects was determined against the native substrate, in vivo [3H]-TG radiolabeled lipid droplets, obtained from control and AKH-treated insects. The lipase activity of the system composed of AKH-cytosol and AKH-lipid droplets (11.1 +/- 2.1 nmol TG/min-mg) was 3.1-fold higher than that determined with control cytosol and lipid droplets (3.6 +/- 0.5 nmol TG/min-mg). Evaluation of the role of AKH-induced changes in the lipid droplets on lipolysis showed that 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 the cytosol. However, the phosphorylation level of the TG-lipase was unchanged by AKH, indicating that phosphorylation of the TG-lipase plays no role in the activation of lipolysis induced by AKH.
(c) 2006 Wiley-Liss, Inc.