20-hydroxyecdysone Reduces Insect Food Consumption Resulting in Fat Body Lipolysis During Molting and Pupation

Abstract

The insect steroid hormone 20-hydroxyecdysone (20E) acts through a specific nuclear receptor complex, ecdysone receptor (EcR) and ultraspiracle (USP). EcR and USP are FXR/LXR and RXR orthologs, respectively, which play critical roles in the regulation of lipid metabolism in mammals. Lipid concentration in Bombyx hemolymph and lipase activity in fat body peaked during molting and pupation, suggesting that 20E induces lipolysis at these stages. Differing from their mammalian orthologs, the 20E-bound EcR-USP was not able to directly stimulate fat body lipolysis in both Bombyx and Drosophila. Instead in Bombyx, 20E slowly reduced food consumption and then induced starvation, resulting in fat body lipolysis. Molecular analysis revealed that the evolutionarily conserved adipose triacylglycerol lipase gene Brummer was transcriptionally up-regulated by 20E-induced starvation during molting and pupation. To our knowledge, this is the first report demonstrating that the steroid hormone 20E is a critical regulator of lipolysis in insects.