20-Hydroxyecdysone-induced transcriptional activity of FoxO upregulates brummer and acid lipase-1 and promotes lipolysis in Bombyx fat body

In a previous study, we have shown that the molting hormone, 20-hydroxyecdysone (20E), reduces insect food consumption resulting in fat body lipolysis during the non-feeding molting and pupation stages, and assumed that the transcription factor FoxO is involved in this process. To verify this hypothesis, we cloned foxO from the silkworm, Bombyx mori. During molting and pupation, FoxO is highly expressed and predominantly localizes in the nuclei of fat body cells. 20E induced foxO mRNA expression and FoxO nuclear localization resulting in an increase of FoxO transcriptional activity. RNAi of foxO prior to the 4th larval molting downregulated two lipase genes – the insect adipose triacylglycerol lipase homologue, brummer, and an acid lipase, acid lipase-1, in the fat body. Overexpression of the constitutively-active form of foxO (foxOCA) upregulated brummer and acid lipase-1 in both the fat body and Bombyx Bm-12 cells. Putative FoxO-response elements (FREs) are present in the promoter regions of brummer and acid lipase-1, and mutation of the FREs attenuated their FoxO-induced luciferase activities. ChIP assay revealed that FoxO binds directly to those FREs. Moreover, foxOCA overexpression in vivo doubled lipid concentration in the hemolymph, increased total lipase activity, and slightly but significantly reduced lipid content in the fat body. Taken together, we conclude that 20E increases the transcriptional activity of FoxO which, in turn, upregulates brummer and acid lipase-1 and induces lipolysis in the Bombyx fat body during molting and pupation.